Note that there are non-zero integer types that can also be used in this way, like NonZeroU8 https://doc.rust-lang.org/std/num/type.NonZeroU8.html. The NULL pointer is also used as a niche, and you can create your own as well, as documented in https://www.0xatticus.com/posts/understanding_rust_niche/
But yes, there are the NonZero integers, and you can make your own NonBlah integer using the "XOR trick" for a relatively tiny performance overhead, as well as you can make enums which is how the current CompactString works.
The link you gave mentions that Rust does this for other types, but in particular OwnedFd is often useful on Unix systems. Option<OwnedFd> has the same implementation as a C file descriptor, but the same ergonomics as a fancy high level data structure, that's the sort of optimisation we're here for.
Alas the Windows equivalent can't do this because different parts of Microsoft use all zeroes and -1 to mean different things, so both are potentially valid.
(And I think for much the same reason, the niche optimization. Option<bool> is 1 B.)
(And for the non-Rustaceans, the only way to get a bool to be not false or true, i.e., not 0 or 1, would be unsafe {} code. Or put differently, not having a bool be "2" is an invariant unsafe code must not violate. (IIRC, at all times, even in unsafe code.))