According to Newzoo, US players of Fortnite, PUBG and Apex Legends are 71% Console gamers, 17% PC, and 12% Mobile (although expenditure from each is not clear, by the way).
If this is correct, it could mean that contrary to the belief of many, iOS didn't make Fornite a success, nor it is responsible for Epic's major source of revenue. Even if Mobile (not only iOS) is the largest user base for potential users.
This would explain why Epic is using this tactic at this time, given that just recently the major tech companies are being put under the scope for their practices (that's debatable), so Epic is suing the company responsible for the smaller source of their revenue given the love/hate for such a massive company like Apple, so the risk is "minimum".
If Epic wins by allowing the iOS be open to other forms of installing software, this would make a precedent of then being able (by suing others with the same principle) to make Console platforms be able to install software from a source different than their stores, for example.
Epic is already doing their Store on PC because the rules there are different, but this would allow them to then have their own stores in each platform.
It's nothing personal, is just business. Citing "in retaliation" in their campaign against Apple, they're trying to shift the perspective and opinion, even if they did indeed break their own agreement beforehand on purpose.
If this stands for a real appeal or not remains to be seen, but I really doubt it, but something clear is that Epic is not doing this at this time on a random date, it's a calculated effort to well, make more money, in which case can you really blame them? At least they planned ahead with their counter suing, but I'm not sure if they saw coming the fact that Apple is removing ties with all Unreal on Apple devices if they don't change their mind. Again, it's nothing personal, just business.