The point is that Mozilla wants to use their own browser engine for Firefox on iOS for all users. While the change opens up the possibility to do this in the EU, it's only possible to do so in the EU. iOS users in the US cannot get this version.
This means that Mozilla would have to maintain 2 versions, one for non-EU using webkit, the other for EU using their own, preferred browser engine.
This means that Mozilla would have to maintain 2 versions, one for non-EU using webkit, the other for EU using their own, preferred browser engine.