Certainly the DU isn't a companion and won't join the party if not selected as a player character, and so in that sense isn't like the other Origin Characters, and is far more customisable as we can choose their name, race, class, appearance, etc. Personally I don't think it follows that it's not an origin character, and that if Larian want to define an origin character in BG3 as any character we can play that has a predefined story that we can discover then that's okay by me, but I agree it's good to be aware that there are important differences between the Dark Urge and the origins like Astarion, Shadowheart, etc.
And I agree that the DU does seem to have more in common with characters like BG1's Charname, DA:O's origins, and even Shepard, than the other origins do. Based on what we've heard so far, the DU might be a less flexible character than they are, but certainly much more than the other BG3 origins and less than a custom Tav. And presumably introduced as a middle ground for just that reason, for those players who gave feedback that they wanted to create their own character yet still have the game tell a really strong story about them.
