This is precisely how I would recommend doing this if I were to be asked. Because git is content-addressable, it lends itself very well to being stored in an object storage system (as long as it has the right consistency guarantees). Instead of using CephFS, they could use Ceph's rados gateway which would allow them to abstract the storage engine away to working with any object storage system.

Latency and consistency would be my concerns - S3 does not quite have the right semantics for some of this, so you'd have to build a small shim on top to work around this. Ceph's rados doesn't even have these problems, so it is quite a good contender.