Color or other styling can replace underlining. But it should always be made visually unambiguous what is a hyperlink/button and what isn’t.

That's true for some sites, and that's what Wikipedia does. But in apps, I don't mind if some discoverabilty (such as for non-essential links) is sacrificed - people will eventually learn if the app matters to them.

Take the case of HN. The "15 minutes ago" and the username are both clickable for a topic. But not underlining or differentiating them makes the text more readable. Most of these things are opinions of course.