I'm not a lawyer either, but the GPL doesn't say anything about commercial use. Zipo can sell the code in the apps without having to ask permission from contributors. The only restriction is that they have to keep their modifications open source (which that Github response says).

But the main point is that the Zipo people bought out the Google Play listing, giving them access to the millions of users who have those apps installed on their phones. They likely don't give a shit about features/keeping the apps closed source. It's just a purchase of the userbase, likely for shady reasons.

This also means that forking the Simple Mobile repos isn't even likely to accomplish much. Sure, it'll put control of the repo in the hands of a more trusted party (which is significant), but since it's open source anyways, it'd be easy to catch any attempts to sneak malware into the apps. And if all development effort moves to the fork, Zipo can still take that fork and redistribute it under the "Simple Mobile Tools" name.

Ultimately, the fucked up thing here is that the original developer, Tibor Kaputa, sold out millions of users. Forking isn't going fix that. Fuck him. The only thing that will fix this situation is if Google takes down their store listing, but that's not going to happen. Hopefully F-droid does.