Well that's true and has been pretty glaring, but they've needed to do that in cases where models seem to fail to grasp the some concept across the board and not in cases where they don't.
Like, every time an LLM gets something right we assume they've seen it somewhere in the training data, and every time they fail we presume they haven't. But that may not always be the case, it's just extremely hard to prove it one way or the other unless you search the entire dataset. Ironically the larger the dataset, the more likely the model is generalizing while also making it harder to prove if it's really so.
To give a human example, in a school setting you have teachers tasked with figuring out that exact thing for students. Sometimes people will read the question wrong with full understanding and fail, while other times they won't know anything and make it through with a lucky guess. If LLMs (and their vendors) have learned anything it's that confidently bullshitting gets you very far which makes it even harder to tell in cases where they aren't. Somehow it's also become ubiquitous to tune models to never even say "I don't know" because it boosts benchmark scores slightly.