Unless you've heard somethingI haven't, I think your'e overestimating what "interaction" means here. I don't think they'll physically show up and talk, at least not outside of cutscenes. I think there will be warlock dialogue options where we can ask for help and our patron will magically whisper in our ear, no need for any physical appearance at all.
Well they are NPCs confirmed to have some interaction with the player warlocks...how extensive we don't really know. But they absolutely CAN be defined NPCs that exist in the game and could be encountered in all playthroughs by all characters. Will that happen, hard to say, but it certainly is a thing that is possible.