The issue is, however, that the parties (customers) haven't received any copies or rights under the GPL2 as due to ubiquiti's license violation, ubiquiti didn't (and does not) possess the any right to the original software (including redistribution).
Think about the licensing issue as a licensing chain / tree with each version having a separate license (instance).
And that's exactly what that paragraph avoids. It stops the revocation of the license tree at the first party who either correctly ships the full corresponding source, or doesn't redistribute the work at all. Basically those parties get a license directly from the copyright holder.
Think about the licensing issue as a licensing chain / tree with each version having a separate license (instance).