For sure some cars use hydraulic brake assit instead of vacuum assit for the brakes (my 2001 Mustang GT did for instance). However, the reports I read indicated that the Lexus in the California wreck used a vaccum booster (and the Lexus parts catalog supports this).
Hydralic type boosters depend on the engine running to provide hydraulic pressure. You can include an accumulator in the circuit to provide back up boost for sutuations where the engine dies, but this is still limited.
I am not sure what the hybrid vehicles use. A hybrid you can't depend on either engine vacuum or an engine driven hydualic pump to provide brake boost, so I assume they have an electrically driven hydraulic pump to provide the boost but I don't know for sure. Maybe they use the ABS pump....
Yes. I guess even trained professionals can panic.
Ed