I just had a look at the Adobe stock licencing and royalty terms and I'm not 100% clear that they are bulletproof either. They certainly don't mention AI training in their terms (I guess they argue that this would fall under the "developing new features" part of the clause).
It does seem however seem a significant change of the terms, as Adobe was essentially paying contributors based on their downloads (i.e. how often their work was used), but the whole AI model is they use the work to not having to pay the contributors anymore.