There are multiple continuous safety checks on such a system, and there was no risk of injury here. The fan itself is constantly monitored. Much more importantly, the pressure on the exhaust chamber is constantly measured (which can catch things like blocked outlets, a fan that might be spinning but not effective, etc).

If the exhaust fan couldn't maintain that negative pressure after the user stopped spinning it, the furnace would turn off again.

Their hack worked because the fan couldn't get the initial inertia up to speed (bad capacitor, dusty bearings, etc), but could maintain speed once it gets there. Have you never had an old home fan that would just hum when you turn it on but then work fine if you gave it the original crank? Same premise.

There was no risk here. If the fan didn't spin up to speed after that initial manipulation, and didn't constantly maintain the necessary flow, the furnace would have turned off again.