Someone was looking into this recently, as in two days ago:
I was hoping that the presence of some additional chips on the board would give a clue as to whether or not I2C was used vs UART. I have a ZyXEL GS2210-8HP that has an ADuM1250 Dual I2C Isolator which I was suspecting might be an indicator that it used I2C to communicate with the STMicro STM32F100.
The GS2210 series uses the same firmware as the GS1920 series. At least according to the switch wiki. Since the GS1920-24HP uses I2C, I think it's likely that the GS2210 will do so too.
One thing to bear in mind, is that the BCM591xx PSE chips also use I2C to communicate with the MCU. The power domain isolation on a board could be placed between the SoC and MCU, or between MCU and PSE chips.
If the Soc is using I2C to talk to the MCU shouldn't the MCU show up somewhere when using i2cdetect similar to what you found on the Unifi Switch Lite 16 PoE?
It should show up, yes. As long as you can provide a driver for the I2C master, at least. I'll have to update the PoE overview page too, as I've listed the USW Lite 16 PoE as using UART. Somehow I had a trace file titled "UART", but the (my) device documentation clearly says I2C...