And the practice in question here is... A statement we haven't seen?

The FTC defines anticompetitive practices as:

Anticompetitive practices include activities like price fixing, group boycotts, and exclusionary exclusive dealing contracts or trade association rules, and are generally grouped into two types:

• agreements between competitors, also referred to as horizontal conduct
• monopolization, also referred to as single firm conduct

I don't think you can rightly say that making a statement is itself worthy of being called monopolization, even if we actually knew what was said.

That's literally all the article is. Matt Booty said something that some people suing Microsoft want to claim is indicative of their intentions with regards to the Activision deal, which at the time the thing was said was still several years in the future.