It wasn't a challenge at all, he was trying to kick the ball, but came in far too late, hence his foot landing on Onana's shin. I'm not saying it wasn't a mistimed "challenge" if you can call it that, and obviously a lot of people seem to think he should be sent off (although they all seem to be Everton fans. I haven't heard any non-Everton fans saying it was a red card). If you go in with force into a player's shin, or higher, there is a fair chance you'll damage them, hence Sue Smith's "leg breaker" comment. However, there wasn't much force in the initial challenge, the main element of the foul was when he then stood on Onana's foot, which is what the booking was for.
The Trippier challenge was much worse and a clear red. It was a scissor tackle with force, and could easily have caused a serious injury.
It was clumsy, and there couldn't have been huge complaints if he'd been sent off, but I'm not sure it now gives players carte blanche to stamp on people's legs.