After emailing them, they did indeed confirm that the 500 PQM minimum does not (any longer?) apply to Thai Airways. Kind of annoying that these kinds of things aren't explicitly mentioned on each partner's page (and that you have to interpret no mention as that it is not done).
Thankfully United still give 100% PQM on most Thai Air fare classes (well, thankfully on the discount fares - it seems a little cheap on the biz-class fires).