They are mad they will have to maintain 2 browsers (one for the EU and one for everywhere else), that's it. I'm not sure what they expected. I don't doubt that Apple will be forced to allow third-party browsers in more regions now that this exists. Now countries can just point the DMA (or parts of it) and say "That, I'd like that please" and Apple can enable it in their region (I know it's not that simple but it is built now, more countries are going to require it assuming the EU experiment doesn't fall flat on its face).
Of course we are still waiting to see if the EU accepts Apple's proposal. Anyone that's ever submitted an app for review (to Apple or Google, but let's be honest more-so for Apple) know this feeling of uncertainty and it's a bit a schadenfreude in that for sure. As much as I like Apple's products I can't but hope they get a few bad "reviewers".