Broadly, yes.

The groundwork for this kind of "matching" process is: sourcing odd versions of the obscure tooling that was used to build the target software 20 years ago, and playing with the flag combinations to find out which was used.

It helps that compilers back then were far less complex than those of today, and so was the code itself. But it's still not a perfect process.

There are cases of "flaky" code - for example, code that depends on the code around it. So you change one function, and that causes 5 other functions to no longer match, and 2 functions to go from not matching to matching instead.

Figuring out and resolving those strange dependencies is not at all trivial, so a lot of decompliation efforts end up wrapping it up at some "100% functional, 99%+ matching".