We generally understand a statement if it is similar to another. For instance, ‘I’m hungry’ is similar to ‘I’m thirsty’ and any of them helps us to understand other statements concerning our needs.
What’s about the statement ‘all the existence is an illusion’ which is evidently spoken by an existent person? It should be at best similar to a statement by which someone speaks about himself as a non-existent person.
Certainly, such a person will seem insane to others. Though, we meet this kind of statements in testaments and their authors may be reasonable persons. We admit their statements as reasonable too, because we know they will become available in the future.
If we apply the same pattern, then we might admit the sentence ‘all the existence is an illusion’ as available. Because soon or later, those who utter it will lose the reality of now existing things, since they will die.
It’s true, we have only personal experiences of dying which make the sentence about the universal illusion acceptable. But for the reason that anybody dies, we should accept it as an ever available way of interpreting a side of human reality.