I thought this about a year ago when they came out of stealth, however; Rivian has all the magic in their sleeves.
1: They actually did the work, the prototype trucks are rather far in development for their announcement. Seemingly they are almost R&D-complete. This is different from most other EV startups like FF or Lucid as they pretty much just showed off mockups.
2, and this is what really matters:
They got major investments from established ICE manufacturers (Huge investment from Ford) with agreements to cooperate on building the factory and making products together. This is the real barrier to entry on EV startups and they executed on it by spending years figuring out the powertrain issue and then going off and getting help by those with the institutional knowledge on manufacturing them. This is why I think they will win.
3: It's a van, a skateboard with a metal box on top, likely borrowing heavily from already developed designs. Note that the van appears largely production ready lacking all the normal concept drawing details and lacking any form of wild 'FUTURE!!' details. Boring Ford van mirrors, standard marker lights, correct size tires and rims. This is a production-ready design, not a concept-car, that likely makes use of the Ford production machine for all the little bits. Think of it like building an app with Angular or React, they know what they want to do and what they want to use other sources for.