It has been tacked on as a "totally not an integer" type because the integer system isn't extensible enough to accommodate it.
It could of course be added with an ABI break (bumping intmax_t to int128), but that's not being done because it would be an ABI break. Hence, not extensible in practice.
It could of course be added with an ABI break (bumping intmax_t to int128), but that's not being done because it would be an ABI break. Hence, not extensible in practice.