It feels to me that "symbolic assembly" is simply creating an AST for the low-level target, instead of just stopping the use of ASTs in the intermediate forms.

It feels to me that "symbolic assembly" is simply creating an AST for the low-level target, instead of just stopping the use of ASTs in the intermediate forms. The larger projects out there might not necessarily go in that direction, but this is very common in smaller utilities that focus on handling byte/vm/machine code. I could be missing something, but that seems like the TL;DR to me. Once you have easy AST nodes representing the lowest output level, it's simple to manipulate, especially in a Lisp-derived language (which is always a plus in metaprogramming).