I'm pretty willing to believe what they say about heat signatures. Hot air has a way of escaping. A couple minutes after an alarm goes off, there's got to be heat showing SOMEwhere, even if there's not necessarily a lot of smoke yet.
911: what's your emergency?
Homeowner: I called 30 minutes ago for a firetruck because of an electrical fire in my basement, where are they!?
911: Oh, we sent a drone to look at your house, it didn't see any fire from the air.
Homeowner: Well my basement is still full of smoke, and I can hear electrical arcing
911: Can you see smoke or flames from the outside of your house?
Homeowner: No, just the basement
911: Wait until the flames have burnt through the roof or walls of the house then give us a call and we'll send another drone. If we see a fire at that time, we'll refund the $99.99 "false alarm" fee from the first drone. Please make sure that you really see flames this time, as you only 3 false alarms before we stop sending out the drone. Those things are expensive to operate, you know.... maybe go down and try fanning the flames to see if you can really get the fire going you call us again.
If the experts say you can affirm where there's a fire or not the vast majority of the time, I'm inclined to take their word for it, especially if (going back to triage) there's more fires than manpower at the moment and the opportunity cost of making sure is measured in lives lost at another call.
Have any fire fighting experts claimed that you can reliably detect an early stage house fire with a drone? Will you be as inclined to take their word for it if you call in a fire, and the fire department says they couldn't see it from the air, so you must be lying about it?