It's not really true in the United States either. NASCAR is the most popular motorsport by a decent amount, but not to the point where F1 and IndyCar are irrelevant (and F1 is way more popular today than it was 5 years ago in the United States).
NCAA Football and NCAA Basketball are all very popular. They have events that are very much mainstream popularity despite everyone knowing they are way worse than the NBA and NFL.
AEW is popular and most people who watch wrestling do acknowledge it on a commercial level. Many wrestling fans prefer AEW over WWE, it's big enough to at least be a debate like Sega vs Nintendo.
That is also ignoring that America obviously is not the world. Japanese baseball is mainstream. Baseball is more popular in Japan than it is in the United States, so it doesn't make much sense to say that only super hardcore people watch Japanese baseball. It's very selective.
MMA is a global sport, and the top leagues are trying to be international. There very much is room for multiple top tier sports leagues especially in MMA. This already exist in many of the world's most popular sports, more popular than football, baseball, and hockey.
NCAA Football and Basketball are popular within the states and has gained popularity in parts of Europe and Asia.
AEW was more popular a few years back when they actually started taking fans away from the WWE and made Vince take notice and turn his show up a notch. As of late AEW shows kinda suck they might put on just 1 good PPV per years whereas a couple years back every PPV was unpredictable and crazy.
F1 has been popular in Europe and Asia for decades but over the last couple years F1 popularity has grown very big in those countries. The USA basically ignored F! for decades as they were pushing NASCAR and Indy car racing. The states could no longer ignore F1 the past few years due its popularity has grown across the pond. The past 2 years F1 has gained many fans in the states. F1 2023 has been a record breaking year for attendance everywhere they have raced so far drawing over 400,000 attending some events over the weekends. In Miami the attendance has grown to over 270,000 that race officials already announced they will be adding another 100,000 for the 2024 GP.
This is why I've said many times the MMA Org that will put a dent in the UFC and will draw fans away in the states will be the Org that is very popular in Europe and Asia. We've watched MMA Orgs to this day trying to succeed in the USA and they fail. The best way to succeed in the USA is to become the biggest MMA Org in Europe and Asia and it will draw the attention of MMA fans in the states. Its human nature how people will want to see something when that something is very popular elsewhere.