There isn't a person on this board that can provide proof of anything behind the scenes that involves the NBA. In most cases, "evidence" is merely what the media, government, or in this case, the NBA itself, tells you. All of the former lie, have lied in the past and will lie in the future.
If you yourself cannot prove that the NBA doesn't manipulate the lottery and I cannot prove that they do and the only evidence is the NBA telling us that they don't, does that mean you are right because you have evidence of the NBA saying they aren't manipulating it?
Some funny stuff has happened. DAL traded one of the best players in the league to the NBA's favorite team (despite them claiming to not be biased) without even so much as contacting another team to see if a better deal was on the table. Then DAL goes into the lottery with slightly less than 1 in 50 chance of winning the lottery and they win it. All of that together is incredibly suspicious. All of it is within the realm of possibility as well.
I don't personally have enough proof to believe in it one way or another. The NBA has a lot to lose if they get caught manipulating their lottery but they also have nothing to lose by providing transparency into the lottery and they've never done that from what I've seen. They have us in a spot where we merely have to go off their word. I find' that suspicious. I don't consider the word from liars to be evidence. If you do, then that's what I consider to be naive.