The issue is more than they can’t avoid using AV1 and are therefore forced to license their patents for free to the rest of the consortium while AV1 probably infringes.
Note that the EU isn't claiming that the standard itself is bad. They're saying the cross-licensing terms are anti-competitive. The cross-licensing may have made sense for major companies like Apple, Google and Meta but it significantly disadvantages smaller companies which may need to rely on their patent arsenal as a deterrent but also face economic pressure to adopt the standard.
This whole thing seems to be a breeding ground for submarine patents, where in the end nobody has an actual applicable patent to show but the thread of pulling one out of the hat is there to make things worse.
They have to join AOM if they want to use AV1 and they need to use AV1 if they want to be relevant because all the big players banded together and made that unavoidable. That’s the heart of the anticompetitive argument.
Most companies don’t exist to sue for patent infringement. They are not patent trolls. The point of competition law is that they should be able to do business without having to go to court with a conglomerate.