That is completely false.

Typically I saw 30% to 100% performance improvements on ext4 by deleting and restoring database directories.

You can see disk fragmentation on linux with the filefrag and other commands.

Are you trolling? NTFS.

Modern file systems are better at _avoiding_ fragmentation than FAT was, but they are not immune to it.

They do, it's just that Windows runs defrag automatically as a background task nowadays so you don't notice.