load of rubbish. if you train combat sports you train combat sports. You have a lot of people in MMA w striking backgrounds that get outstruck and plenty of striking experts that come to MMA and get KO'd.
Cerrone was supposedly 25-0 in Kickboxing and had a title yet got tooled by people like RDA, Pettis, Diaz, Even barbosa was lighting him up. Varner dropped him, so did Mccollough and even the other Cowboy was doing well before it hit the ground.
MMA fighters can box.... kickbox.... etc. Just go watch Mousasi, Overeem, Hunt, in K1. I believe even genki sudo did some k1 with some degree of success and you've had just so so so many mmartists with boxing careers that were succesful that failed to capitalize in MMA striking....