I don't know...? I have a boss whom I call "Boss" but she didn't like the nickname at first. It warmed over time but it wasn't an immediate willing reception. It just kind of stuck.The thing about a nickname is that it's received either willingly or unwillingly, and it says something about both the giver and the receiver. Clearly it was received willingly, in that Picard had the power in the relationship, but I find it hard to imagine him doing so. Even if he was being friendly, I would expect a "Please call me Jean-Luc."
It's possible to invent potential scenarios when this could have come about, but that's the same as making up stuff to explain away Discovery's plot holes. Just not my bag.
Life is weird. I don't need to invent "possible scenarios" when I live them.