[aborsy]

This is just repairing the index file, automating what used to be done manually: salvaging the rest of the data that has not been lost hopefully (not restoring lost data).

[amano-kenji]

Does restic not try to fetch the pristine data chunks from the source?

Technically, there are two copies of data. One is on the source. The other is on the destination. Thus, restic can be like raid1.

Restic has checksums for chunks. RAID1 with checksum can repair errors.