Rampage is the number one movie at the box office and is a huge hit for Warner Bros. Inside the new movie you'll find a reference to another big Warner Bros. movie, which is, coincidentally or not, made to an interesting character. The line comes from Naomie Harris who, after she and Dwayne Johnson get picked up by the U.S. Government, are introduced to the character of Harvey Russell played by Jeffrey Dean Morgan. Russell is being cagey about exactly what branch of the government he works for, and Harris' Dr. Kate Caldwell throws out a few potential organizations like the FBI and CIA, before making a snide remark and asking if Russell actually works for the Justice League.
DC Comics is owned by Warner Bros. so the reference to the Justice League, rather than, say, The Avengers, makes sense. Odds are that when the line was included in the script the expectation was that the Justice League movie was going to already be a massive hit, which would make the joke extra funny. It didn't quite work out that way, but the Justice League is certainly still well known enough for the line to work.
Of course, there is an extra added bit of "in-joke" in the reference as the recipient of the line, Jeffrey Dean Morgan is part of the DC film universe, as he appeared in the role of Thomas Wayne at the begging of Batman v Superman: Dawn of Justice. The actor has also made no secret of his desire to play a larger role in those films, as he is very interested in actually playing the part of Batman if and when a Flashpoint movie ever happens. Depending on how such movie might go, it's entirely possible that we could see Morgan as a member of the Justice League at some point, making the joke work even better retroactively.
Of course, Jeffrey Dean Morgan isn't the only one in Rampage with a DC movie connection. At some point, Dwayne Johnson himself will be appearing in that universe as Black Adam, though exactly when that will happen is currently unclear. It doesn't appear that the character will appear in the Shazam movie that's currently filming, though that movie might set up Black Adam's appearance down the line.
We never do learn exactly which government agency Russell works for. It could be anything. Although we know he's not one of the Men in Black, as the rights to those movies are owned by Sony. The impression we're given, however, is that he works for one of those agencies so secret you've never actually heard of it, thus his interest in giant animals destroying cities. To be fair, it does sound like the sort of thing the Justice League could have dealt with too.