I think that's a myth that needs to die. Let's look at a top stadium champ in Muay Thai along with a couple MMA champs who have the best takedown defence in the game along with a solid kicking game of their own.
Nong-O fighting MT in small gloves
Jose Aldo had very strong kicks in his prime and is still nearly impossible to take down
Joanna is a very high volume kicker and she's almost as hard to take down as Aldo
The stance is actually quite similar, it's not nearly as different as most people think. A good stance for kicking is also a good stance for defending takedowns, there's no real need to use a modified wrestling/boxing stance which compromises kicking, but that's what most MMA fighters end up doing.
There's really nothing that stops an MMA fighter with good kicks from launching full power kicks into an opponent's arms until he breaks them.