There’s no rules for the Fediverse, all it means is that they utilise the ActivityPub protocols to be able to federate with other websites that also use it (there’s others, but basically irrelevant now).
Mastodon requires OAuth2 for apps to get access to your account because it was designed that way, and Lemmy wasn’t, it’s as simple as that. Any platform can be part of the Fediverse (including Reddit, Twitter, Facebook etc if they really wanted to), which also means that platforms can also do anything they want.
You can have links open in NewPipe by setting it in Android’s settings:
Settings -> Apps -> All apps -> NewPipe -> Open by default
And then set things up in there