It makes perfect sense that LLMs are good at this. We've always known that a binary is not a true moat, but that it's just a lot of toil to transpile.
Compiler output is highly predictable, but it doesn't make the inverse true. LLMs are not themselves compilers.
It's easy to train LLMs to decompile code. All you need to do is take the source code from existing training data and compile it. Then you swap the order and show it the compiled code, then the original, and judge it by how well it predicts the original.