Sure, no one needs that source code anyway because they are not adding anything of value. But if they are not complying the GPL license they are legally not allowed to use the GPL projects.

As an example for their relationship with upstream, they market the btrfs feature (https://www.synology.com/en-global/dsm/Btrfs ) but don't have anyone working on btrfs (and btrfs could surely need the help). I can see one patch set from them which hasn't made it into mainline ( https://patchwork.kernel.org/project/linux-btrfs/list/?submi... ).