If I play chess, and someone changes the rules so pawns can now move in any direction, and rooks can now move diagonally, and kings can now move any number of squares, am I still playing chess?
That said, if you say to the person you are playing against, "But those are just optional rules. You don't HAVE to play it that way. I am, but you don't have to. It's up to you if you want to challenge yourself that way or not."
Do you think the other player is going to not use the new rules when the entire game is now designed around the new rules?
Comparing chess to dnd is apples and oranges. Chess has a set of rules where you HAVE to play that way or else you can't play the game. In DnD the rules are there as a guide. If not a guide and you HAVE to play 5e the way it is, then homebrew rules would not exist.
