I'd argue it's more similar to 64-bit x86, and the only difference between the "standard" x86_64 ABI and x32 ABI is that x32's pointers are 32bit.

Everything else matches the 64-bit version - not the 32bit one - the instruction encoding, the extra registers, the minimum isa extensions (x86_64 puts a baseline of sse2), the calling convention advantages etc. etc.

There are a lot of future-oriented decisions in the x32 ABI, for example 64-bit `time_t` to avoid the Y2038 problem.

In part yes, the difference is the ability to access the 64-bit registers, the additional set of registers, and the assumption that SSE is present. But because of all of these, x32 defines a new call convention to optimize function calls.