Synology is a bad example. Many of their custom syscalls are related to filesystem things like btrfs raid5 self repair and carefully removed from their gpl source code. Ianal but imho this is a gpl violation.
Though it probably is a violation, because they probably don't do that.
It's misleading, I'll give you that, but let's be pedantic (lawyers are)!
They can probably provide the missing bits in dead tree letter format upon written formal request received at their post address.
It does look like Synology are not abiding by the GPL here, but a minor nitpick: GPL v2 has no such requirement to distribute the source with the binaries. But if you don't, you do have to tell people they can get it and provide it in "machine-readable" format on request.
See https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html section 3b
What you're describing would likely not pass as being a reasonable interpretation of the license.