Before understanding why intelligence is illusion you first need to understand illusion concept very well.
Illusion - it simply means something which appears to exsist but in reality it don't. Example - like a lie.
Why intelligence is illusion?
First you need to understand intelligence. As I said intelligence is nothing but your ability to understand things. And how one can understand things? On based of experiences right? Without no experience one cannot understand anything.
So our intelligence comes after experiences. It means our intelligence is completely on mercy of experiences. Imaginary example - our world have law that if you throw something up it will come down right?
But what if there is other world in which if you throw rock then it flows up instead of coming down? In that case both perspectives are correct but they are based on their own personal experience.
This is the reason why I said intelligence is also illusion because one day it could appear in one form but we have no idea that it might appear totally different from its previous form. In such case one time it could appear as our world is only truth and other time it could appear that it is not.
So what appears to us is changing because experiences is also changing. This is why nobody can claim a truth through their Intelligence because intelligence is also illusion. It is false it depends upon experiences which is changing. And a thing which change cannot be the truth.
How come oneness is then ultimate truth isn't it also a perspective of someone?
See that is why I keep telling that this subject of oneness isn't about intelligence if you try to understand it then you will fail. This oneness is subject of becoming which you already are. How can you become which you already are? Ask these types of questions to yourself not me. It will help you to remove your ignorance.
And about how is can be truth? So this thing that everything here is simply existing or everything here is simply a presence. Example - trees are presence, rocks, mountains, you, your surroundings people. Everything is simply existing. So this presence or existing is oneness. This is the other reason to call it oneness too. See everything is one? Isn't it ? More easy - everything is simply exsisting, so saying that everything is one, it is not wrong. This fact of simply knowing that you are exsisting is known as awareness. But that doesn't mean that tree is not itself exsistence. Just like how exsistence you are, the tree also.