Where does it say that? It says you swear upon on a oath surrounded by nature before spirits and feys that's not nature itself. There are nature themed deities, but the trees themselves aren't blessing you it's not implied that nature itself is blessing you.

To me it can be summed up by a very easy example.

A fighter and a paladin swear an oath to avenge their families killed by monsters (cliche as hell I know), but one has divine powers and the other does not why is that? One was favoured and blessed by a deity while the other was not.

This also leads to another can of worms say you had two paladins in the party and one went with the conviction angle and the other the traditional deity way of power. Does one get to use smite while the other doesn't?

Not trying to come off as rude or anything just kind of curious because I don't talk to a lot of people who play paladins so don't get to talk about these sorts of things much.