I believe boot chess also does not implement two of those three, which makes it a fine comparison. Promotion is something I expect most people know, castling is something they might know, but en passant is hardly common. I wouldn't be at all surprised if the number of people who play chess without knowing that rule exceed those who play by FIDE rules.
Whatever the case, it is still a variant of chess, so I see no problem calling it chess.
Promotion shouldn't be compared to castling/en-passant here.
A game without castling or en passant would be similar enough in spirit that we could call it a "chess variant". Removing promotion, on the other hand, would result in a wildly different game.
I actually suspect it'd be impossible to win against any non-beginners, making it more similar to tic-tac-toe than to chess.
It would be more like moaning that he broke the four minute mile but collapsed fifty feet before the finish line. Should it be counted if you didn’t finish?
This program is awesome, but it’s not quite chess.
> It was the smallest implementation of chess on any computer until its record was broken in January 2015 by the PC-compatible BootChess.
with this:
> 1K ZX Chess uses only 672 bytes of RAM,[2] but implements chess rules except for castling, promotion, and en passant, including a computer opponent.
If you don't include castling, promotion, and en passant, the game isn't chess anymore, or at least it hasn't been for a couple hundred years.