> https://isocpp.org/files/papers/D3935R0.html
In section 2.1.1 above, there is "we don't consider std::float32_t to be C23's _Float32"
Is this correct?
Yes, otherwise any requirement in C regarding _Float32 would automatically apply to std::float32_t as well, and that's not the case.
https://eel.is/c++draft/basic.extended.fp#7 recommends _Float32 as an additional name for float32_t
It's not the same as float, even if float is IEEE binary32.
You're reading too much into it. This paragraph just recommends that the keyword names "should be chosen to increase compatibility and interoperability" with _Float32 et al. An implementation could also choose a different name, and even if it chose _Float32, that doesn't necessarily mean it's the same type with same semantics as the one in C.