As long as people in the US can't test their web app on "firefox for iOS" without first buying a plane ticket to the EU and getting an EU sim card, all eu-only browser engines on iOS will be second-class citizens.
I think the next logical extension is that actually limiting general public use across the entire world makes apple less compliant with the DMA. Mozilla will not be able to justify putting significant effort into the iOS port as long as it can only reach a small fraction of users, so in reality the way to get browser-engine competition in the EU is to mandate that apple _not_ impose EU-specific rules about what apps can be installed.