No, that is not true.
There are two concepts here: being close to reality and being certified for training. They are not the same thing.
Certified is in essence an insurance policy. It means "sue me if you find my simulation responsible for a crash, because I am so sure it is accurate". That is why it is so expensive. But just because something is not certified to a high standard, does not mean it is not accurate. It just means no body has spent the resources to verify its accuracy because there is no economic benefit of doing so.
the manufacturer keeps those details more or less classified so the
developers don't even know what to simulate
Well, not when there is an agreement between the developers and the manufacturer. For example Boeing has a relationship with one of the companies in the aircraft simulation industry. The simulation is so detailed that you can execute abnormal checklists using the real Quick Reference Handbook.
And there are things which do not require propriety materials from the manufacturer. On the other end of the spectrum, a Cessna 172 for example, is so simple and so widely used that it is a matter of how many resources one is willing to put in to get the simulation right. There are products which simulate the combustion engine down to cylinder stroke level, and the inner workings of a combustion engine are well understood.
flight simulators (...) are about as close to the real plane as Call
of Duty to real warfare.
Even PC-based flight simulators can be used in real pilot training. For example it is useful for learning IFR approaches. Surely certain aspects of the simulation are not as real, but that does not prohibit it from being used as a training aid. Again, note that I am saying it is helpful, not that you can log IFR hours by flying in the simulator. And there are simulators which you can log IFR hours on; they are based on the PC-based software which I am talking about.
Let's talk more about certification. There are various levels of certification, covering different aspects of the simulation. For example there are simulators which focus on navigation and instrument procedures, and they are certified by the aviation authorities for such tasks. They are not, however, certified for practicing say an engine bleed failure.
The Level D simulators, which presumably your commercial pilot got his training in, has a very high standard and low tolerance. They are called "Level D" because there are Level A, Level B and Level C simulators. Level A is more relaxed and does not require a motion system for example. Level D is at the top of the game and they are the very best simulator one can get their hands on.
every single flightsim just a simple game that bares a slight
resemblance to reality but that's about it
You can take the below statement as a personal account or as a bluff on the internet: after my first introductory flight at a flying school, the staff asked if I have heard of PC-based flight simulators and suggested that I learn and practice some basic stuff in the sim while pursuing my PPL. That was before she learnt I would do barrel rolls and hammerheads in the sim...