I will acknowledge that this doesn't necessarily answer the question about Altavault.. there are also data escrow options available for AWS from third parties, but they don't necessarily integrate with Altavault data shards.
But more generally, for primary data access, FAS NAS snapshots are essentially WORM from a client point of view - you can't modify them from client systems, but as you recognise, you can delete them from the cluster's admin interface.
For managing secondary copies on FAS, we have Snaplock functionality available on FAS as well - this technical report outlines the options available - this prevents even compromised admin accounts from damaging snapshot copies. We also have functionality in the new ONTAP 9.3 to support two factor authentication (2FA) providing for non-repudiation of admin actions, and even deleted volumes, if noticed quickly, can be recovered in some circumstances.