As for what MCU does stand for in the X-Men universe, my guess it’s Mutant Containment Unit, or something along those lines, since Professor X, Magneto and others are being held captive. Deadpool 2 featured a similar group called the Department of Mutant Containment, which handled security at the Icebox prison. Maybe the MCU is affiliated with the DMC, or maybe this is a separate group altogether that’s substituting for the DMC. These timelines are so confusing. In any case, these MCU operatives have apprehended some of Dark Phoenix’s main characters (I wonder if they’re working for William Stryker), and I don’t think they’re going to let the mutants watch Iron Man.
Article source: https://www.cinemablend.com/news/2467675/did-you-spot-the-mcu-labels-in-the-new-dark-phoenix-trailer-heres-what-they-could-mean