Apparently it's not public yet.

I'm 99% sure it's their usual BS about "security" and "safety".

Most likely they will attack the EU's definition of the relevant market as being "iOS App Store" and claim they only have a sliver of the larger "Mobile app store" market, even though iOS and Android apps are not substitutes for one another. How you define the market is usually the crux of antitrust battles.

The European legal system has not been captured like the US was by Robert Bork (yes, that Robert Bork) and the Chicago School (dogma: monopolies are a logical impossibility because markets are infallible). Thus this will likely only stall for a few months, I'm sure Apple knows that, but it's a few more months of those juicy App Store margins that are so essential to Apple's financials under the "Services" rubric.