If it wasn't a boss you wouldn't get loot drops, the first Vosik part is an encounter and your loot box would only grant you the siva keys and a chance for exotic
How does it not mean anything? You get rewards from a boss fight. Zamboni crushes you boss kills you. You don't put the parts in, it wipes you, boss kills you. Vosik part 1 is an encounter, you don't wipe you have all the time in the world to lower his health, no loot drops save for the chests which is a random chance
It doesn't mean anything because it has the same principle. Same thing with Daughters, Totems, and Oracles, all which dropped loot and all which had to happen to have a time limit yet they weren't considered to be Bosses.
While you are correct about the fact that it will not have a challenge mode, your justification isn't quite accurate. The siege machine satisfies the Outbreak Prime's quest step #8. Research and Destroy; "Defeat the Devil Splicers' leaders in the 'Wrath of the Machine' Raid."
Technically, it is a boss, as it also spawns the key chests like Vosik and Aksis, while an encounter (Phase 1) does not.
Just saying.