The “All” is quite clearly referring to the Upanishadic “All”, or Brahman. Note as well that the Kaccānagotta Sutta makes use of sabbaṃ, which is translated as "the All" and not sabba.
Kaccānagotta Sutta“This world, Kaccana, for the most part depends upon a duality—upon the notion of existence and the notion of nonexistence. But for one who sees the origin of the world as it really is with correct wisdom, there is no notion of nonexistence in regard to the world. And for one who sees the cessation of the world as it really is with correct wisdom, there is no notion of existence in regard to the world.
“This world, Kaccana, is for the most part shackled by engagement, clinging, and adherence. But this one with right view does not become engaged and cling through that engagement and clinging, mental standpoint, adherence, underlying tendency; he does not take a stand about ‘my self.’ He has no perplexity or doubt that what arises is only suffering arising, what ceases is only suffering ceasing. His knowledge about this is independent of others. It is in this way, Kaccana, that there is right view. ‘All exists’: Kaccana, this is one extreme. ‘All does not exist’: this is the second extreme”
Sabbaṃ is a noun. Its the name of something. I wonder what this can be?
Brhadaranyaka Upanisad“1.4.10. Verily, in the beginning this world was Brahman. It knew only itself
(atmanam): "I am Brahman!" Therefore it became the All. Whoever of
the gods became awakened to this, he indeed became it; likewise in the
case of seers (rsi), likewise in the case of men. Seeing this, indeed, the
seer Vamadeva began:-
I was Manu and the sun (surya)!
This is so now also. Whoever thus knows "I am Brahman!" becomes this
All; even the gods have not power to prevent his becoming thus, for he
becomes their self (atman).
So whoever worships another divinity [than his Self], thinking "He is
one and I another," he knows not. He is like a sacrificial animal for the
gods. Verily, indeed, as many animals would be of service to a man,
even so each single person is of service to the gods. If even one animal
is taken away, it is not pleasant. What, then, if many? Therefore it is
not pleasing to those [gods] that men should know this.
1.4.11. Verily, in the beginning this world was Brahman, one only.”
A further interesting elaboration from “The Early Upanisads - Annotated Text and Translation”:
Now we do find sabbaṃ in SN 12.15, so could they be talking about the same thing? If we look at the only other place where we find sabbamatthi" (the All exists) and "sabbaṃ natthi" (the All does not exist) we find SN 12.48:“4.9-10 the Whole: the exact sense of the term sarva, here translated as "the Whole," has been much debated. As Gonda 1955a has shown, the term in its earliest usage did not mean "everything" but carried the sense of completeness, wholeness, and health. It is, thus, opposed to what is partial, broken, sick, or hurt. In the Upanisads the term is used to indicate not all things in the universe but a higher-level totality that encompasses the universe. Gonda (1955a, 64) observes that the phrase sarvam khalv idam brahma at CU 3.14.1 does not mean "'Brahman is everything here,' but 'Brahman is the complete here, this whole (one),' or: 'Brahman is what is the whole, complete here, is what is entire, perfect, with no part lacking, what is safe and well etc., i.e. Completeness, Totality, the All seen as the Whole.'" Unless the context dictates otherwise, I translate sarvam throughout as "the Whole" and the phrase idam sarvam as "this whole world." To the English reader the term "whole" should evoke the senses of totality and completeness (all there is), as well as perfection, soundness, and wholesomeness.”
https://suttacentral.net/sn12.48/en/sujatoAt Savatthī. Then a brahmin who was a cosmologist approached the Blessed One … and said to him:
“How is it, Master Gotama: does all exist?”
“‘All exists’: this, brahmin, is the oldest cosmology.”
“Then, Master Gotama, does all not exist?”
“‘All does not exist’: this, brahmin, is the second cosmology.”
“How is it, Master Gotama: is all a unity?”
“‘All is a unity’: this, brahmin, is the third cosmology.”
“Then, Master Gotama, is all a plurality?”
“‘All is a plurality’: this, brahmin, is the fourth cosmology. Without veering towards either of these extremes, the Tathagata teaches the Dhamma by the middle….”
This matches the pre-Buddhist cosmological debate in the Chandogya Upanisad 6.2 -
transl Olivelle, p.247“In the beginning, son, this world was simply what is existent—one only, without a second. Now, on this point some do say: 'In the beginning this world was simply what is nonexistent—one only, without a second. And from what is nonexistent was born what is existent.' 2"But, son, how can that possibly be?" he continued. "How can what is existent be born from what is nonexistent? On the contrary, son, in the beginning this world was simply what is existent—one only, without a second. 3 "And it thought to itself: 'Let me become many. Let me propagate myself.' It emitted heat. The heat thought to itself: 'Let me become many. Let me propagate myself.' It emitted water. Whenever it is hot, therefore, a man surely perspires; and thus it is from heat that water is produced. 4The water thought to itself: 'Let me be- come many. Let me propagate myself.' It emitted food. Whenever it rains, therefore, food becomes abundant; and thus it is from water that foodstuffs are produced.”
Given then that the Kaccānagotta Sutta is actually about "the All" existing or not in terms of pre-Buddhist Upanishadic cosmology, of which the Buddha was surely aware, and given that in other suttas the Buddha does say that things exist via the the use of “atthi” its clear that the Buddha did not reject all ontological commitments. Therefore, Nāgārjuna falls. The Dhamma is not about surmounting all views of existence and non-existence and sadly, I'm sorry to say, Ñāṇananda does too.