(Also PlanetScale employee here) Each shard finishes a backup at it's own time T, so two different shards could finish minutes apart (or even more, depending on the difference in size). for pretty much any use of a backup, you'll effectively be doing a PITR, not a raw restore from the cluster. The PITR timestamp is what unifies all the clusters together, regardless on when each backup finishes. think of backups as jumpstarts for actual restores (like for cluster resizes), where WAL replay and replication get the node to real time (or some specific point)

with that said you can restore without PITR if you don't care about synchronization, but generally you'll just use PITR