The pixar docs in a couple of places justify why they went their own geo format rather than extend alembic, they talk a lot about layering, being more than geo, building on innovations from Pixar and other developments since alembic etc.
Amusingly when it comes to volumes, they just went 'oh, yeah, we'll just do a thin wrapper around vdb'.
Volumes are VDB
Yep. Put down a volume lop, it expects a path to a vdb on disk. Easy. Not much to say here really, except maybe a leading question about how sequences are supported, so we may as well talk about animation next.