Unless something changed dramatically at GitHub they're a 100% solid-state fleet since 2012 when they started down the "building their datacenter cages" journey.

Whilst you could say as the 500lb gorilla in the market they can afford that luxury, there are other examples, i.e. DigitalOcean, where SSD is a core part of their product and it is offered inexpensively ($5/mo).

Even if you're looking at 3-way replication of your data, meaning you're buying 768TB of storage, the SSD should only tot up to $200-300k for Intel S3510 (or similar).

There's a non-trivial amount of additional effort involved in any kind of "multi-layered" storage (c.f. data on 7200rpm, NVMe cache) over and above just making all of your storage moderately performant in IOPS terms. That has cost too.