To the best of my knowledge, this was an accusation made by Kasparov after he lost the game, there is no actual evidence of this happening - or if it was within the rules of the tournament (to tweak). As you might know, "Deep Blue" was more than just software, a big part of deep blue was its massive parallel processing power crunching literally 200 million nodes per second (nps or positions per second), even though source code of deep blue is not available to public, raw logs of the game are available and nothing from those logs suggest that any monkey business was going on (it has been debunked many times).

I think Kasparov got overwhelmed by the numbers that DB was crunching and he lost the game psychologically even before the game started. If you actually studied the games he played against DB - you would see amateur mistakes made by one of the world's greatest grandmaster.

I think if he knew he wasn't playing a computer, he could have done a lot better.

Regardless, there is no comparison between deep blue and modern computer engines. To the best of my knowledge, most (popular) comp-comp tournaments and comp-human tournament, the binaries of the engines are given to the tournament organizers which are used in all games throughout the tournament. This is most definitely true for comp-human games; for comp-comp games it might depend on the participants and the organization hosting the tournament.