Luca's point was that defrag is unlikely to be useful for ReFS when block cloning is in use simply because it's impossible for a single block to be in an optimal location for more than one file. Simple example:
File1 -- |Block1|Block2|Block3|Block4|
File2 -- |Block5|Block2|Block6|Block4|
In this example, File1 has four blocks of 64K each (cluster size) laid out in order on the disk, so 0% fragmentation. File2 is sharing two blocks from File1, so it is immediately 50% fragmented. There's simply no way to avoid this when cloning the same blocks between files, there's nothing defragmentation can do about this, if I defragmented File2 then File1 would be fragemented.
At this point I'm not recommending file system defragmentation runs on ReFS until more testing is complete (I haven't found it to cause any issues, but so far also no benefit, just wasted I/O). However, if you are using one of the forever modes, such as reverse incremental or forward incremental without synthetic/active fulls, then using the Veeam settings for defrag and compact can still be useful to clean unsused space and reorganize the metadata within the VBK file.