Let me quote the person I was replying to for you:
Exactly like you do if you want to play the original Doom with modern source ports. Since the game is not available to buy, this means either pulling those assets from an original retail copy, or pirating them.
They are clearly stating that Doom is also not available to buy, which is not true.
In the post you were replying to, perhaps inserting a paragraph break before "Since the game..." might help, more than selectively quoting from it. It might then be more clear that the phrase "the game" is referring to the same game both times it's used.
Let me quote the person I was replying to for you:
Exactly like you do if you want to play the original Doom with modern source ports. Since the game is not available to buy, this means either pulling those assets from an original retail copy, or pirating them.
They are clearly stating that Doom is also not available to buy, which is not true.
In the post you were replying to, perhaps inserting a paragraph break before "Since the game..." might help, more than selectively quoting from it. It might then be more clear that the phrase "the game" is referring to the same game both times it's used.