I'm honestly surprised how brazenly they are doing exactly the thing they had 10 years of legal trauma from previously again. I mean, something different, sure. But exactly the same thing????!
77.5% is definitely a "dominant market position". That is the term that matters, people just like to use the word monopoly instead (note: if lots of people use a word "incorrectly" the meaning of the word changes).
End result: Lots of big, successful companies that do shady things and only the EU intervenes and then is the bad guy.
Look at any of the threads about fines for Facebook or the like and you will find many replies to the effect of "The EU is just jealous and should not fine successful American companies".
America has the same laws, basically, as when Microsoft was found guilty.