There's multiple issues here that apparently others aren't seeing.
The key one: it's a build or buy decision for a non-key part.
SpaceX's USP (as far as I'm aware, not a rocket nerd) was reusable rockets. If they can successfully demonstrate that, then they've won. They don't want to run out of money before then but chasing down every inefficiency in the existing system seems more like OCD than good business sense.
Subsidiary issues:
Sending the guy who "had never built anything" to negotiate hardware orders.
Getting the guy who "had never built anything" to build a vital part when off the shelf alternatives were available.
Celebrating this as business genius not unnecessary risk.