Yes, you’re both correct:
You need one sensor for each area you want to monitor independently. With devices more capable than the ESP32‑S3, the coverage would likely be greater.
The ESP32‑C6, in particular, offers significantly better performance. Check out this comparison video from Espressif: https://www.youtube.com/watch?v=JjdpzM6zVJ8
How about if I want to monitor the apartment as a whole? Would 100m2 be too much for one sensor to handle?