an obscure clause in TOS won't be a small print of an evil villain speech exposing their plot in clear wording. what it would be is something worded vaguely enough to make things seem like the end user technically agreed to what was being done, it could also be an "and" where you expected "or", or an ommision of a specific thing... my point being - it's always going to be a technicality that in case of a lawsuit would be a valid defence in the eyes of law
it very much is how it works though? show me a lawsuit someone lost before they got caught commiting a crime. and how would you even go about proving that your unpublished documents were used to train AI? even an entire life's work of one person is just a speck in the training data, it's impossible to definitively prove your work was stolen and used to train an AI. besides there will always be plausible deniability that the AI just made shit up that happened to look kinda like what you once wrote