Are silicone socks safe? It's fairly common for E3D to sell silicone socks for their hot ends. There are also other companies that sell these silicone socks for their hot end cartridges.

According to a brief internet search, it seems the ignition temperature of silicone is surprisingly low - around 450&nbsp;&deg;C. This surprised me because I was under the impression silicone would just burn  / evaporate if it were heated up to a much higher temperature.

If my thermistor/heatrod slips off, my heatrod will glow into an orange temperature during thermal runway. This only happens briefly, but its color indicates it is reaching a temperature around 790&nbsp;&deg;C. 

So, are silicone socks safe? Couldn't they ignite fairly easily?
Silicone socks are safe to use, provided your printer is safely operating and you are using the silicone  socks in their operating temperature range.

Your current setup is NOT SAFE!

When the heater element falls out of the heater block (that should not happen in the first place, please secure it correctly) and heats up to about 800 °C this means that the printer has no active Thermal Runaway Protection (TRP) enabled. Basically, when the thermistor doesn't measure a temperature rise while the voltage to the heater element is being scheduled, the firmware should shut down the voltage to the heater element. When this fails, the heater element can reach dangerously high temperatures to start burning anything that can catch a flame on touch. In deliberate tests, heaters have been able to melt the aluminium of the hotend:

                
            
You should be worried at this point as you see from the link you provided what happens if the heater is not shut off when the heater element is disconnected from the heater block:

Please fix your printer ASAP by uploading a proper firmware with enabled TRP protection before proceeding to print anything or at least don't let it print without supervision and proper smoke/fire detection devices.