I think it's rather that they're still forced to use Webkit everywhere but the EU and financially it probably doesn't make sense to port Gecko to iOS when worldwide iOS has pretty small marketshare. iOS only holds over 50% marketshare in USA, Canada, Australia, Japan, Norway, Sweden, and Iceland. In most EU countries it's under 30% marketshare. Android literally dominates in all of Mexico, South America, Africa, and Asia, in other words, far too many countries to list, most of which iOS has less than 10% marketshare.

Why would you go through so much trouble for arguably such a small slice of the world's iOS market when the majority of the iOS market will still be forced to use Webkit?