The default English translation should remain as "hotter" (I know it sounds unnatural) because Duo makes two sentences (IDN/ENG) as a pair. If Duo changes to "higher", no one can answer from English to Indonesian. However, "higher" should be also accepted as an alternative answer as other people mentioned.
FYI: I got answers from native speakers whether they actually use "panas" (hot) and "dingin" (cold) for "suhu" (temperature). All four native speakers said "panas/dingin" are good collocations for "suhu" in daily conversations. "Tinggi" (high) and "rendah" (low) are only for scientific contexts. https://hinative.com/questions/18399309