I'm sure there is truth in original author saying tax code complexity as the core challenge. But that's not what makes this hard. That's domain complexity we all come up against it's accidental complexity that killed the ports.

The real problem is idiosyncratic and esoteric coding practices from a single self-taught accountant working in a language that didn't encourage good structure.

I can translate well-written code without understanding what it does functionally, so long as I understand what it's doing mechanically.

The original author seems to build in the assumption you're not going to translate my code you'll need to rewrite it from the the tax code!