No, but the gap isn't as big as boxers want to pretend. Especially with mma guys with boxing heavy styles, they've had boxing coaches and been training and sparring in boxing gloves for years.
They'd basically look like them face a decent but not great boxer that they had no footage on and didn't take seriously. It's not like the mma fighter is going to come in with slick boxing and shoulder rolling and out skill the boxer, but a good chin and good power from someone you're not that prepared for makes for a clumsy looking fight and a while to find the timing if you do at all.
Anderson beat Chavez Jr at 75 years old, Mayweather took several rounds of barely throwing anything before he got Conor's timing, Fury looked terrible. The boxer will probably have one of their worst performances against an mma fighter, but that's still usually enough to win.
It's also way overplayed how poorly boxers would do in mma if they were fighting guys with punching heavy styles. Mercer KO'd Sylvia in 9 seconds when he was a recent champ and had only lost to top 5 guys, plenty of kickboxers have come in and learned some grappling defense in almost no time.
