That's for the Atari parts using linear addresses. There were numerous third-party memory expansions that used bank switching as in the XL's to get much more. I wrote Point-of-Sale software using 128K banked expansion for an 800.

I also had my 400 upgraded to 48K with a mechanical keyboard.

Yeah, the 16KB cards were from Atari and the banked switched cards were from third parties. Antic Magazine (named for the chip) had a lot of ads in the back for those.