I'm not so sure they'd be put into D3. Before last night, they were #5 in NCS, all divisions (according to MaxPreps).
They will likely drop a spot or two on the next set of rankings, but certainly not much further than that.
Lots of games left in the next couple weeks, including one each against the league's Big 3. NCS Open is still not out of reach for the Mats, and they'd have to get upset at least a couple times to fall all the way to D3.
At least, that's the way I understand the rather convoluted set of rules... And, of course, I don't know which of the rankings sets they use anyhow.
One thing I did spot: Open teams that lose before the section final get another game, which I'm assuming would be for placement/seeding in NorCals in whatever division the CIF chooses for them.