Apple's rationale in their letter about this was essentially that they would need to ensure that it works (create and review use of a framework for non-Safari-b

Apple's rationale in their letter about this was essentially that they would need to ensure that it works (create and review use of a framework for non-Safari-backed PWAs in iOS) and that (usage of PWAs) x (percent of userbase affected) was too small for them to bother.