Do you have any evidence to show there was no icing at FL80 or FL150?
Your suggestion that the level was smooth is a good one. The METAR showed BKN CB at 3300 and overcast at 10,000. Smooth air would be above the cumulus layer and FL80 would be below the overcast. FL150 would most likely be above a stratified overcast. So I think that it is a reasonable assumption that they were not in icing.
However, the 737 should be able hold in light to moderate icing for two hours without any issues whatsoever.