It's called "semifluid" because the fuels in question are viscous and/or not even in the liquid phase in the first place. Ever seen crude oil? Not anywhere near as liquid as water. IRL you can also have things that are technically solids but can be pumped around as if they were liquids, for example because they are a very fine powder, or for reasons even weirder.
Granted, it doesn't make sense for gasoline (BC fuel) and ethanol, which are not the least bit viscous. But I suppose they wanted to avoid making a block that only burns one single thing (crude oil) that has better uses elsewhere (refining). Game design trumps realism in some cases.