I thought by filters they quite literally mean the antivirus filters in Defender? Does it strip down some of the Windows file semantics too?
File system filters are pluggable kernel drivers. For example, ProcMon (Sysinternals tool) monitors file systems via a file system filter -- so fun fact, if you outright disable file system filters on a ReFS volume, you won't get any ProcMon results! This was a 'duh' moment for me.