4. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
As I read that, as long as you don't further redistribute, you should be good.
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.
4. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
As I read that, as long as you don't further redistribute, you should be good.