Apple has not made any rule modifications public. The correlated activity from Google and Mozilla may indicate that the two companies anticipate Apple to soon relax its restrictions on alternative browser engines, or they may simply be hedging their bets. Many of Apple’s App Store restrictions are being loosened as a result of regulatory pressure from numerous governments, including the (reluctant) acceptance of third-party payment services, sideloading of apps, and third-party app stores.
However, that might be altering. The Register reports that Google and Mozilla have recently been seen collaborating on Chromium and Firefox builds that utilize their respective standard Blink and Gecko rendering engines.
Although the iOS versions of Chrome, Firefox, Edge, and other browsers can currently sync with their desktop counterparts and display any user interface they choose, the WebKit requirement means that most of their strengths and weaknesses are the same as those of Safari. On macOS, there is no such restriction, so third-party browsers are free to use any rendering engine they like.
The amount of storage that these browsers are allowed to use for caching content, the amount of memory and CPU power that they are allowed to use while running in the background, how aggressively tabs must be removed from RAM to make room for other apps, what extensions they are allowed to use, and a host of other restrictions are all still theoretically open to Apple. But for the iPad specifically, allowing third-party browser engines on the platform should lead to more third-party browsers that resemble their macOS and Windows equivalents in terms of appearance and functionality.