Sounds really hair-raising! First of all, I would say: DON'T SIGN ANYTHING!
Regarding the controlled residential ventilation, what salespeople tell you is partly incomprehensible, I’ve experienced it too, and I was almost convinced. But then I managed to do a turnaround and decided on a controlled residential ventilation system. And I’m glad about it. The actual purpose of controlled residential ventilation is fresh air. In winter, it’s cold outside, so it’s nice to automatically have continuous fresh, preheated air. In summer, the controlled residential ventilation runs at minimum. I still don’t have to ventilate after showering; I go straight to work. And if it’s not too warm outside yet, we have the terrace door open all day. Or the bedroom window.
What’s important is not to reach the "value Kfw70," but how this value is achieved!!!!
Instead of a solar thermal system, for example, you can achieve (approach this direction) the value by increasing insulation. Controlled residential ventilation lowers the value as well, just like switching from gas to a heat pump.
Whether it makes economic sense is another matter.