(Disclaimer: I wrote one of the answers)
Given that all answers are free to quote and link sources, I think it's fair to let the votes decide. Would it be better if all voters were educated in German labor law and had this specific case been decided in court? Sure. But that's not how this site works, because there would be no site then.
I think the other answer would have more credibility (and votes) if it actually cited primary sources. All it does is reiterate hearsay, and it even gets the small details wrong that are easy to look up and fact check (45 vs 60 minutes, 11 vs 12 hours). I'm not saying it's factually wrong overall, but if it lacks sources and has mistakes in the parts that are dead easy to verify, then it's hard to trust that the main argument is sound, just based on "believe me, someone biased told me this is true".
I quoted the law. Anyone who thinks my interpretation is wrong can just downvote the answer.
You can also open another question somewhere (don't know if law.SE would find it on topic) to ask if there are court decisions on this. I have certainly seen scumbags try that interpretation, but I have never seen it reach court because people (like the OP) would either say "bye, I'm going to get a better job" or "damn, I have to eat this, I cannot get another job". So if you can find a primary source on this, I'm happy to read it. All we have now are people saying "but it happens!". Yes. It does. Robberies also happen. Every day. Does not make it legal.
As far as the theory that the guy misunderstood something? Maybe. It's as good a guess as any, but having seen German working contracts, I doubt that the company provided them with the office hours, but not the working hours. It's not impossible, but it's certainly less probable than a "start up" trying to get their employees to work overtime. So I can only take the OPs word that the question as asked is actually their question.