Also modders had reversed all relevant code at some point (at least to the extend of this function does X) and then offered those symbols (=their class names, method names,… And where to find them in the bytecode) for others to mod the game. Later Mojang even released the official symbols, but at that point everyone was already using the community made ones. IIRC they kept releasing these symbols for every new version since then which drastically sped up modding compatibility for those versions.

So I think it’s not completely unexpected that they’d do this since the most important information was already oit in the open.