The author links to a post which indicates that while ext4 with 256b inodes might technically store mtime with nanosecond granularity - the call that ext4 to ge

The author links to a post which indicates that while ext4 with 256b inodes might technically store mtime with nanosecond granularity - the call that ext4 to get that time is limited by the scheduler clocktick granularity, aka HZ which is 100, 250 or 300 for most people, hence reported granularities of 10ms, 4ms, 3.33ms, etc.