I heard that rumor. If true, I don't find it particularly hard to believe.
Meliodas reigned Supreme among his clan members before he could even become King. I've already talked about how Liz was likely at a point his equal. Mael might've truly been the strongest combatant during the Holy War, equal to or superior than Zeldris with the DK magic—though it's possible that Mael is similar to Zeldris in the sense that he might also be able to siphon power from another source.
Also Meliodas latest talk about the curse gives me the impression of possibly others that have the Might of Kings.
To begin with, these King lvl individuals are not God. They can still lose. The infighting amongst the Demons is pretty denotative that Might is very much regarded, and people would still oppose the King, attempting to dethroned him. Speaking of, Sr Gowther might have had the ability to also oppose Kings. According to Meliodas, he was so regarded by the Demon King for his ability. He even denied the DK of his desire to have him swear fealty to him. The most interesting part is that he even strategized on how to end the Holy War. Dr Gowther is old and wise enough to not be ignorant of what he was up against nor braggadocios of his own power; yet when Zeldris shew up, his only real regret was that he would have to utilise a lot of magic power to take him down—meaning it was feasible in his eyes, though very draining.
Anyway the point is that a few more could have the Might of Kings. The EoS SDS could all, miraculously, have the Might of Kings one way or the other (they're MCs & predestined for greatness). King Arthur could have the Might of Kings as he would apparently surpass all the SDS, including Escanor who might already possess the Might of Kings at 12:00pm.