[KennyBlanken]

They were being polite, as all service workers have to be. Tell me you've never worked s service industry job without telling me you've never worked a service industry job....

There are numerous safety interlocks in gas furnaces ranging from flame presence sensors to (usually) several heat limit switches, often which are wired directly in line between the gas solenoid and the furnace controller. The controller's logic has a lot of "expect this condition by this amount of time" rules, too.

The furnace's controller is usually separate from the system controller (the box that talks to thermostats, valves/dampers, pumps, etc.)

On forced hot air systems that are new enough, the furnace and system controller monitor outlet air temperature and shut down if it's too hot.

Gas furnaces are incredibly safe.

Also, the quote to install the system specified what thermostats they were going to install. It's on you to read the quote, not to get uppity at service personnel and lecture them people haxx0ring your furnace.