“how much experience with ice would he have had flying in the tropics??”
At 30K+ feet the outside air temperature is well near or below freezing even if you’re flying over the tropics.
3 degrees per thousand feet is the general rule of thumb. Not quite straight line, but decent approximation. Likely 65 below above 30000 as a general rule. Thunderstorms thrive in an unstable lapse rate so this specific situation would be different. But well below freezing.