Remember that Apple doesn't allow other web browser engines in any application. Imagine if an Electron application on iOS just decided to ship with it's entire, own browser engine. We would have super-bloated apps like our desktops, and if a security flaw was found, good luck getting all those apps to update.
In a way, Apple knows that allowing other browser engines means:
1. Chromium-based browsers just increase their monopoly, Firefox continues stagnation by almost all odds
2. The Electron mess on our desktops will claim new territory on our phones and bring security problems with it
Not very appealing. Limiting competition? When your only realistic competition is Chrome with it's 80%+ dominance, I don't think a regulator would have a problem with that.