Exactly. Champion Kawasaki (Costa Mesa) used to race at Riverside, and I was pals with a couple of the mechanics. I asked that "break in" question, and the pit boss said something to the tune of, "do you think we ride our race bikes slow on the street for 500 miles to break them in before each race?"
These are lawn mower engines on governors. However, this is on the 212 Mega Max Owner's Manual:
BREAKING IN YOUR MINI BIKE To protect your new engine, enjoy optimum performance, and extend the life of the vehicle, break in your mini bike properly. To do this, avoid full throttle starts and rapid acceleration for the first fifteen miles.