The problem with TKD is the "above the waist" kicks and that messes with the training regime. Unless TKD guys adopt low kicks a lot, they might offer a challenge to muay thai. but muay thai generally attacks the legs first and mixes up with high kicks, whereas TKD guys generally kick above the waist exclusively and they don't know what to do once someone attacks their most valuable asset--- their legs, their weapon.
This is just my opinion and research through watching many fights and studying the rule sets of the respective sports. In a street fight with no rules, meaning TKD guys would train against low kicks, punches, elbows, knees, and also train on how to do the mentioned techniques, then they may have a chance.
Just my opinion, not fact.