The reasion why this wouldn’t work is very likely that SMB, as a protocol that originates from Windows, doesn’t support file names with backslashes because on Windows certain characters like the backslash are not allowed in file names.
It’s funny I originally disregarded this knowledge seeing I can create a\b under /mnt/remote which made me think this is possible then. The problem which I see now is that in the fstab the mount point is in fact /mnt/remote/subdir not /mnt/remote/ so creating directory under /mnt/remote I was creating it under local btrfs filesystem and not the mounted cifs filesystem…