I feel like games-as-a-service has really messed with people. Outriders is like Monster Hunter, really. The whole game, the whole experience, is there and you can do the whole thing solo if you want. You can, however, also bring friends or ransoms along and there is obviously some end game content for you to play alone or with other people as well.
When they say it's not a games-as-a-service title, they really mean that it's not like Destiny. You're not looking at five to ten years of content updates with battle passes, new areas, storylines and classes where the game you've just bought is basically just the tutorial that'll end rather abruptly at some point and ask you to wait for the next big content drop.
Play Outriders from the beginning to the end, maybe stick around for a while to kill stuff with friends, but the main story won't continue until Outriders 2 - whenever that comes out.