But why did he conclude the computer has seen a very complicated sequence that Kasparov himself can't even begin to see?
The reason is rather obvious, IMO. Kasparov assumed, with good reason, that a computer is not likely to make tactical mistake. Especially in an end game the computer can see x moves deep perfectly. Computers historically are far weaker strategically -- doing things like creating good pawn structures.
So when Kasparov saw a move that appeared to be a tactical flaw his first belief wasn't, "Oh, must be a bug in the program, that was a random move." He thought, "There's an attack vector here I'm not familiar with."
It would be like sitting with John Carmack in a coding session and suddenly he deletes a function that seems like it would be useful to you. Your first thought would be, "Oh, John must have realized this function isn't useful. He's smarter than me. I just don't know the whole story." You likely won't think, "Must be a bug in John's brain that on occassion he will randomly delete a function."
So what you're saying is that, because Kasparov assumed the move was anticipated further than he could see, chess's "nature" is inferior to go, because in go–you implỵ–this wouldn't have happened.
No, according to this theory, it is because Deep Blue has a completely alien mind, and of the nigh-impossible to count ways in which that alien mind could work, it behaved in one of the many ways that Kasparov failed to consider.
Now, it might about the game if complete randomness is a valid strategy in Go.
I'm not talking about obviously bad moves.
I'm not trying to imply that Kasparov is a bad player either.
But why did he conclude the computer has seen a very complicated sequence that Kasparov himself can't even begin to see?
It's because of the nature of the game.
Remember, we're talking about a completely random move here ..