File system filter drives apply to all (RW) file systems on Windows. It's not exclusive to NTFS or ReFS.

Windows has an extensible model. It's a different approach from most (all?) other OSes. It offers a different set of features.

Sure, AV could perhaps be done in a different manner that would be more effective/faster, I can't comment on that as I lack the insight required -- only MSFTies that work on kernel code could respond in any authoritative way.