This feels very close to charging for a system feature (something Apple doesn't allow).
watchOS already has a web browser. It's just not meant to be launched like an app. You only see it when clicking links in messages / emails etc.
What this developer did is basically create a launcher for the built-in web browser, and added a way to navigate to different URLs. So it's like 99% Apple's work, and 1% this developer's, yet they charge for it, and market it like they actually built a new web browser (they didn't). Doesn't feel very ethical to me.
Meanwhile you have legit, hard-working app developers like Apollo's dev who were not able to add push notifications as a paid feature on their app, because Apple rejects it for charging for a system feature (even though renting and maintaining a push notification server has ongoing costs)...
App Store rules are a mess, and aren't enforced uniformly.
In fairness, Apple don’t allow any browser on their platforms but their own. Even Chrome and Firefox on iOS are using Safari under the hood, so even if the developer did write an entire browser stack, Apple would probably reject it.