Nobody denies that. The same can be said for any striking background. That doesn't mean that it isn't a good base however, provided those other skills are learned.
Really there isn't anything technically flawed about Muay Thai that would inhibit a transition. In fact they would have an advantage over other striking backgrounds in close quarter clinched up fighting. What could be argued is that the fight pace would need adjustment, as the first two rounds in Stadium Muay Thai are typically slow paced with a build up to rounds 3 and 4. Most people in MMA come out guns blazing in the first round, but you've seen Thai's that have adjusted to this in Kickboxing.
I find it hilarious that there is an argument that Thai's have tried and failed. We probably can't count beyond 10 that have tried ever, compared to thousands of Japanese/Americans/Brazilians etc, and it's also built into their culture where MMA is non existent in Thailand. I also find it amusing that this argument is specifically Thai Muay Thai fighters versus the world in their respective art. As if you don't see Muay Thai techniques being used in every single MMA card, or non-Thai's with Muay Thai backgrounds that are successful.