If this interview process is so golden (based on your metrics of success) why isn’t every one doing it?
* It doesn’t scale well to huge organisations.
* It’s not standardised “repeatable” in the sense that you cant generate datasets from this that let you correlate interviews with subsequent outcomes and improve hiring metrics company wide
* It requires expertise in pair programming, which most companies don’t have
Basically, OP’s hiring process is an excellent example of “doing things that don’t scale” to gain an asymmetric advantage as a small organisation.
They don’t need to derive metrics, they can invest the time, they have a team who are skilled at pair programming - so they can have this hiring process and other people can’t.