You may well be correct about it being to keep moisture out. Coincidentally, the very next V11 I had with trigger failure did NOT have this silicon sealant, and had corrosion visible on the trigger assembly wiring terminals. When i bent the terminals back to release the wiring, one of them snapped clean off. So I managed to find a used trigger assembly with a broken trigger. Swapped out the broken trigger for one of MV's replacement ones, and job done. Or so I thought: it ran the motor, but the speed control was locked into Medium and couldn't be altered. I suspect that the board was damaged by water getting on it and causing long-term damage.
By another amazing coincidence, I'd just been given a complete motor and cyclone unit by someone who'd got it as a warranty replacement for a broken trigger, so I had to swap out the trigger yet again to finally get a fully working V11. So far, I've only come across this sealant on one machine.
Did Dyson ever modify the trigger design before the V10/V11 went out of production?