Those channels are illegal to use outside of the copyright issues they're intended for. It's not just a free "hey take this down for me, will ya?" button.
If there was an icon, that could have be what was copyrighted. Perhaps it's from the usage of the deb package. The creator of a project that uses the GPL can actually have a download link to that software which is under a different license that is not freely distributable.
The icon is in the repo covered by the AGPLv3. Signal-Desktop does not have a CLA that would enable them to license the built binary anything but AGPLv3, as they do not have sole copyright over the code. They themselves are also bound by the AGPLv3.
