I wonder about the same thing. SYX was hiding in a corner where she could see the destruction. Meanwhile the blue flame supposedly would spread and ignite any place where there's air (if how spark could rapidly developed into big fire when it touched the evaporated alcohol or gas is any indication, it should be fast). But, Sage Lin still have some time to threw a question (and maybe more) before the flame reached his place. So, idk. Maybe her corner is far enough to provide some time for VB to make a fire barrier around her before the blue flame reached them.

Assuming that "just" the smelting area was wide enough for a circle of more than a dozen men that hidden area should be pretty big