A heater produces only so much heat, and the cold garage absorbs it. The colder, the more absorption (sort of).
If the heat passed through the battery first, you'll raise it by a few degrees, like you saw.
If you want it warmer, add more insulation or more heat. Just because it didn't work on its own, doesn't mean it wouldn't work with the bike under a -20F sleeping bag.
-Crissa
