It certainly is different. You can lead a market without anti competitive practices.
However, if you do not have a controlling interest in a market, then you can’t actually be anti competitive in the way that anti trust is intended to regulate.
Anti competitive doesn’t simply mean “try to beat your competition”. That’s what competition is.
Anti trust regulation is intended to prevent monopolistic control of an industry, where consumer choice is limited to the degree that a company can do whatever it wants because they are effectively the only game in town.
This is why the idea that somehow the 3rd largest company in an industry would somehow have monopolistic control over the market is nonsense. They will obviously still have major competition in the gaming industry, but virtue of the fact that two other companies will still both control larger chunks of it.
The argument against this merger appears to be based on the idea that if Microsoft’s game division is even remotely close to Sony in terms of market control (yet still much smaller), then Sony is going to get the vapors and collapse, leaving Microsoft in control of all of gaming.
The mere threat of competition is going to destroy Sony somehow, and so we cannot allow that, because competition will be anti competitive.