> lexer + parser + treewalk interpreter

If all you do is construct an AST and interpret it it's not really a compiler is it. It doesn't compile from source to target. At most you're describing a compiler front end (arguably), or an interpreter.

I would expect:

lexer + parser + AST definition and construction + semantic analysis/type checking + X + codegen to ASM/WASM/C

where X includes

  - definition of an intermediate representation (IR)
  - lowering AST to IR
  - static analysis
  - improvers/optimisation passes (at least some simple stuff)
  - code gen including register allocation

This can still be a simple "complicated" program, but there's more to a compiler than an AST interpreter.

EDIT: I notice that the author of the original article has also started work on an optimizing compiler: https://ssloy.github.io/tinyoptimizer/