Shopify has plenty of resources to run their own internal, ultra secure Rubygems mirror.
I think what is most likely is that Shopify is trying to achieve some security goals in rubygems through Ruby Central. That they put a deadline on funding based on some security guarantees.
Shopify knows they are a high value target. I bet they thought they could muscle for whatever they want using funding money, and didn’t anticipate the mishandling and then blowback.
They certainly have the capacity to run their own full on mirror service, but I doubt they have serious incentive to do so given exciting controls and culture re: Ruby and OSS contributions.