In the ATPL they would be talking about issues relevant at airliner speeds
That's not actually true. The ATPL covers all aspects of flight. As a result, apart from Air Law and Ops Procedures, is not a chore to work through. The exams are a bit of a bind though.
, and yes the temp drop around parts of the wings/airframe could be a few degrees C. But not at GA speeds.
I've gone back to my notes and have spotted my mistake! The figure quote below +10deg TAT (Total Air Temp) which is, due to ram rise, always warmer than OAT. Ice does not form above 0deg. Sorry for the curve ball. Temprature probes read TAT if the speed is suffiently high. TAT = OAT + (TAS/100)^2
Runback icing, icing formed between +10 and -3 TAT, on a Jet is formed, so it says, due the high impact speed of supercooled droplet on the leading edge warming and running back and then re-freezes.
So says my notes!