I won't answer as to if I think that the ascetic Gautama would disapprove of the Mahāyāna, but I will offer vital context. From this, forum readers can determine for themselves if this is reasonable or if it is crypto-ātmavādin junk.
The Mahāparinirvāṇanāmamahāyānasūtra exists in upwards of six recensions. The most infamous is that of Dharmakṣema Bhikṣu. This is the "ātmavādin" recension, and it is likely a forgery produced by the monk in question.
"A 'forged' Mahāyāna sūtra?! Say it isn't so!"
Whether we consider all Mahāyāna sūtras to be forgeries or not, it appears that this monk created a new Parinirvāṇasūtra by combining existing versions together and adding passages inserted from Tathāgatagarbha sūtras and other apocrypha, as well as seemingly-personally-invented passages.
There are two particularly infamous passages in this recension. One equates the "Tathāgatagarbha" with the "Ātman." The second states that the Dharmakāya is the "perfection of the Ātman."
The second of the passages that appear to have been inserted by the monk is actually taken from a different Tathāgatagarbha sūtra, T353, the *Ekayānamahopāyavaipulya, a version of the "Śrīmālādevī Lion's Roar." It reads:
The vijñānakāya has differentiation as its basis. Some see the dharmas as destroyed because they do not understand the continuity of conditionality, and so they tend toward nihilism. The uninstructed cannot see the kṣaṇas alongside the continuity of conditionality, and so they become eternalists. With this interpretation and the other, they differentiate so, and then they become inferior, subsequently maintaining their inferiority always.
Living beings are inverted with their five aggregates. They think that the impermanent is the eternal. They think that duḥkha is bliss. They think that what is anātman is the ātman. They think that impurity is purity. The wisdom of all of the Arhats and all of the Pratyekabuddhas can glimpse neither the Tathāgata's Dharmakāya nor the range of his omniscience. If living beings were to believe the words of the Buddhas, they would arouse thoughts concerning the eternal, thoughts concerning bliss, thoughts concerning the ātman, and thoughts concerning purity. These are not inverted views.
How can this be? Because the Dharmakāya is the perfection of the eternal (常波羅蜜 *nityapāramitā), the perfection of bliss (樂波羅蜜 *sukhapāramitā), the perfection of the ātman (我波羅蜜 *ātmapāramitā), and the perfection of purity (淨波羅蜜 *subhapāramitā). Those who see the Dharmakāya thus see correctly. Those who so see correctly are the true sons of the Buddha, (who are) born of the buddhavacana, born the Saddharma, who have been transformed by the Dharma, who have attained the Dharma, and who reap its benefits.
Apologists for this sūtra say that the Dharmakāya is the "perfection" of the Ātman because it, in truth, is the reality that is sought when the worldlings wrongly conceive of the Ātman. It is "better" than the Ātman, "more perfect," a "perfection" of the "imperfect" or otherwise "flawed" Ātman. Other perspectives say that this is a non-Buddhist provisional teaching addressed to those worldlings who are not yet ready for the true "selfless" Dharma. This is based upon a passage from the Madhyamakaśāstra wherein Master Vimalākṣa states that the Buddhas teach ātmavāda to the Lokāyata-Cārvākas, who have neither fear of hell nor the drive towards liberation, in order to save them from hell and set them on the path, in a future life, towards the Buddhadharma.
Does this convince? Unlikely. It doesn't help that, as the passage is presented in the forged sūtra, it has none of the contextualizing clarificatory material from its source. Decontextualized, it is ātmavāda without a doubt, and unfortunately it is completely decontextualized as it appears in the recension of the Mahāparinirvāṇanāmamahāyānasūtra in question.
