I think you may either be mistaking how the displacement math is interpreted or formulated. I'm not saying that from a scientific basis but just from observations of my own..
I have friends that blast their extracts in a large shop with all the doors open and they smoke weed steady in there with no explosions or fires. I AM NOT SAYING THATS GOOD TO DO!! So the oxygen must displace the butane with not too much of a an issue with that much fresh air. They don't use fans at all either. I thought they were fucked for doing so but they've been doing it like that now for a month or so.
Well the 75% in a 10x10 room that I mentioned assumes the already present air is displaced by the butane gas with no leaks of butane and of course no pressurization. I would guess that their ignition source (a burning bowl for instance) is probably up too high and the butane is pooling on the floor. With enough floor space it just spreads before stacking up high enough to meet the ignition source. They're taking extra risks that carry no benefit besides extraction on a rainy day. Even extremely small risks compound after enough events and become somewhat likely.
Daub, you can do extractions safely inside just need to use a closed system man there are a ton on the market butane/co2/alcohol all have closed systems
For the most part. If you have an emergency depressurization somehow you're likely going to explode. Closed systems must still have an emergency systems in place in case of such an event. How and to what extent is up to the person operating it, but closed systems greatly reduce your risk, they don't eliminate it.