This is pure bullshit.
Karate has destroyed MT on many occassions and especially so in the holy grail of standup sports, K1. Karate being fucked by MT has been categorically proven wrong in K1.
Aside from all the great karate fighters in K1 like Andy Hug, Glaube Feitosa, Jerome LeBanner, etc. the most dominant King of K1 ever, the true GOAT, is a Karateka. Musashi, a karate specialist, destroyed Muay Thai champion David Dancrade as well as beating pure MT World Champ Kaoklai.
Good MT fighters will beat not so good karatekas, and vice versa. A good fighter can compensate for the styles difference.