Although, logically, I would have thought you DO need to do both Raspberry Pi OS updates and OpenSprinkler Server/OSPi controller software updates *in addition to* the OSPi firmware, I looked and saw no microcontroller on the OSPi hardware… so definitely NOT a ‘dumb question’. Although the word ‘firmware’ usually refers to the portion of a system that runs on the MCU on dedicated hardware/expansion board, I think they might use it differently in the OpenSprinkler world.
Did you ever find an answer to your question?
(I would also like to know)