Yes and no. Disk durability is complex. For now, we want to set conservative expectations while we improve the volume infrastructure.
The volumes should be at least as reliable as an array of magnetic spinny disks.
You can also configure your postgres to use synchronous replication, which I think is worthwhile even with maximally redundant disk arrays. Back in our Compose days we’d see all kinds of unpredictable single node failures, drive corruption, etc. I do not trust single node databases for anything. 
(Link is fixed, btw).