The problem is unless you know what "third-party accounts" means, it's ambiguous as to which account is being connected to and which account is doing the connecting over POP. "Gmail will no longer support checking emails from [thing I'm unclear on] over POP."

The whole sentence hinges on knowing that Gmail itself can (or could) become a client and go out to other email accounts and fetch emails over POP. In my mind I'm wondering, "Are they talking about something like where Slack connects to gmail?"