Great question — answering as the UphillTech team:
Ext4 checksums only cover file system internals and file metadata , so there’s not much to scrub. e2fsck verifies the relevant checksums during its passes; for example, on a file system with a corrupted directory entry: e2fsck 1.47.3 (8-Jul-2025) Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Directory inode 2, block #0: directory passes checks but fails checksum. Fix<y>? All this can do however is update the checksums; it can’t recover corrupted data.
Full answer by Stephen Kitt: https://unix.stackexchange.com/a/807658 (CC BY-SA 4.0).
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