Having recently designed and produced a low volume (<1000 units), low-complexity (2 layer / 30ish placement) board, I was very happy that my relationship with the assembly house was not fully abstracted behind an API! I suspect this was more of an editorial/marketing flourish..

When there was a problem with the SMT stencil over a connector hole that might have hurt durability, the fab's CTO called me up and then worked with the stencil house to add an aperature. We worked together similarly on balancing cost vs quality when weighing numerous otherwise-equivalent board and parts suppliers. And so on..

That said, it's great to see new stuff in this space. I'm very curious to give CircuitHub a try and see how it compares to my favorite (lower-tech) shop..