McTominay is stronger, better in the air and a decent goalscorer. That's pretty much the only things that he's better than Fred at, and they aren't exactly the most important aspects of being a midfielder.
Fred is a significantly better passer (he passes the ball more, plays more long passes, plays the ball forward more often, sets up more goal scoring opportunities and despite all that has a higher success rate) and he wins the ball back more often when the opposition have it. His movement to always be in position to receive the ball when his teammates have it is far better (it's something that is often a huge weakness for McTominay) so it tends to allow the team to be more fluid at moving the ball around.
McTominay has the potential to be a lot better than he has shown so far, and maybe he might end up being better than Fred. But right now it's not particularly close and I can't understand why people put McTominay ahead. He was good tonight, started well against Arsenal in the last match, but had been absolutely terrible for over a month before that ever since his great Leeds match.