Not necessarily. I don't know the context, but bugs/issues reported from users may have been sufficient to patch their code without touching the ROM themselves.

If you read their writeup about fixing the game, the issues appear to stem from it heavily using a texture format that most GPUs don't support natively and almost no games having done much with that texture format before, so losslessly reencoding them was eating truly astonishing amounts of VRAM to avoid load time issues, and they reworked a bunch of memory management things to make it playable without 12G+ of VRAM.

I don't know if there were any other issues, but at least on that one, it doesn't seem like they needed deep knowledge of the game to try reworking it.