It can significantly slow down wild fires which need to heat a constant supply of fuel to ignition temperatures. Wood burns at 180 degrees Celsius, so the difference between 20c and -50c is +160C vs +230C or over 40% more heat.
This is especially impactful when looking at how easily sparks propagate fires across gaps.
A typical wildfire is not putting much of its overall energy output into bringing fuel up to combustion temperature, though. The combustion energy in a kilo of wood is measured in megajoules; the amount of energy needed to elevate the temperature of a kilo of wood by a degree is measured in kilojoules. So while you're right that the energy budget needed to combust new fuel goes up by 40% or so if the fuel is at -50°C, the energy budget going into that is only a percent or so fo the total energy the wildfire is putting out. The rest of the energy beyond what's needed to bring fuel up to combustion point is going into making massive walls of glowing flames, mile-high convection columns of air, sending smoke thousands of miles, etc., etc...
That is, in many ways, precisely what makes it a wildfire, rather than a smouldering ember.
In the limit - when a fire is dying out - then I guess, for sure, changing the fuel temperature can sap a larger part of the energy budget.
The point is propagation speed. A burning tree surrounded by acres of burning trees is largely irrelevant at that point. It might happen to produce spark that travels miles, but that’s relatively rare. Which is why firebreaks can be useful.
This is especially impactful when looking at how easily sparks propagate fires across gaps.