First of all, a more appropiate translation is not "as a truth or reality" but "as real and actual".So when you can't pin down the Tathagata's existence in the present life it is proper to declare the Existence in the present life?
If it is proper to declare the existence in this life even tho it cant be pinned down, why is not proper after death.
Adress this
Second, to understand it, you need to read the whole sutta. There are a lot of suttas identical to that one, making the same case. Please read the whole sutta, stop this habit of reading just the beginning of a sutta or just the beginning of a post on the forum.
The sutta is reffering to the lack of self. He describes how the 5 aggregates that make up the Tathagata (WICH DO EXIST) are not to be regarded as self.
From the same sutta:
“What do you think, Anuradha, do you regard form as the Tathagata?”—“No, venerable sir.”—“Do you regard feeling … perception … volitional formations … consciousness as the Tathagata?”—“No, venerable sir.”
“What do you think, Anuradha, do you regard the Tathagata as in form?”—“No, venerable sir.”—“Do you regard the Tathagata as apart from form?”—“No, venerable sir.”—“Do you regard the Tathagata as in feeling? As apart from feeling? As in perception? As apart from perception? As in volitional formations? As apart from volitional formations? As in consciousness? As apart from consciousness?”—“No, venerable sir.”
“What do you think, Anuradha, do you regard form, feeling, perception, volitional formations, and consciousness taken together as the Tathagata?”—“No, venerable sir.”
To put it more simply: There are just the aggregates, there is no Tathagata. They are not "the aggregates of the tathagata", that is incorrect. There are simply the aggregates. For example, the branches and leaves of a forest do exist. But they are not me, they are not mine, I can not be described in terms of the leaves and branches of a certain forest. In the same way the aggregates should be regarded.“What do you think, Anuradha, do you regard the Tathagata as one who is without form, without feeling, without perception, without volitional formations, without consciousness?”—“No, venerable sir.”