The view of the Free Software Foundation is that the requirement is for the distributor of the software to make the source available, or arrange with an agent to handle the task. It is not acceptable for a distro to point to the upstream hosting of source files, nor is it acceptable for one distro to point to the source code repositories of another distro, barring the existence of an agreement between the two distros that such linking is acceptable.

Canonical/Ubuntu has been somewhat unique with regard to this, as they at one time deemed it acceptable for derivatives to link to Ubuntu’s source repositories and thus avoid the expense and hassle of mirroring sources. I’m not aware of any other distros that have agreed to host sources for their derivatives.

Nonetheless, Canonical is well within their rights to cancel this agreement and given the popularity and the commercial aspects of the Mint project, it is not unreasonable to expect it to carry its own water, so to speak.