Why does mdadm --zero-superblock preserve file system information?
Because zero-ing the mdadm superblock only removes the metadata that describes the raid array, it doesn’t remove information about what is actually on the rest of the disk. This is actually a good thing, as it means you might be able to recover a volume when the array itself won’t assemble for some unfortunate reason.
And to be clear: because you’re creating the array using volumes and not whole-disk, the partition table isn’t touched by mdadm, so zero-ing the raid superblock isn’t going to affect the drive label nor the partition structure.