We also don't hear about those testing positive being quarantined. But clearly they exist and more so daily.

Those forums have may be 100s of participants, or low 1000s. 100000 people presumably come to Thailand per month. So, those forums have less than 1% of the total visitors. At something like 0.1% positivity rate, forums would be expected to have may be 1 person in quarantine (and may be 3-5 if seatmates come into play). That 1 person is probably busy being locked up r/n.