How exactly do you get away with not using the 32-bit subset of AMD64 ASM?

"32-bit x86 code" just means "code that does not use any of the AMD64 ISA." (And at this point, probably SSE2+ and a handful of other extensions.)

GP is correct: anyone that "knows" AMD64 assembly inherently must know 32-bit ASM because one must use the original x86 registers, instructions, etc. for the vast majority of tasks.

People are still constantly running what was formerly known as "32-bit x86 code" in their 64-bit applications.

I read “learned 32-bit first” as “learned on i386 machines”, not “learned what the 32-bit instructions do on 64-bit computers”. There’s a big difference between these two, because the way you write code for a machine that doesn’t do 64-bit is quite different, since you have many more registers, the calling convention is pretty different, etc.