All the Origin companions have to be in Act 1, so if you play them you start in the same place. I hope there are non-Origin companions, too, but his comment is likely about Origin characters.
It might also have to do with the design of having companion quests able to be completed without the companion. In some other games I have played the companion is required to start the companion quest, so if there is a multipart companion quest, the companion couldn’t do the third part without having completed the previous parts.
To your first point... I think everything we have heard so far from Larian indicates that there will be origin companions and storyless mercenaries you can hire/fully customize. But I am with you - I hope that they add actual companions with actual stories that are also not origin characters.
To your second point... I agree, this is one of the reasons why I think you have to choose. In DOS2 you could even miss certain quest points even if the companion was in your party if you didn't talk to someone, do things in the right order or go to the right place. Usually later in the story there was a way to kind of pick back up on the companion quest even if you missed things earlier. But I imagine you are right - this probably dictates the need for a decision.
One more point to add - having a camp full of like 8-12 companions and all of the other people, animals, etc. you pick up along the way would likely be an enormous pain for them to try to sort out. So they likely want to try to control the number of companions lingering around so they can keep adding to the camp as the story moves along.
