I'm doing a study on the epistemology of Greek Fire, and whether or not any of what we know about Greek Fire can be considered knowledge, and also what shall be the criteria for possessing such knowledge. If there are any Byzantine scholars that can offer some time for me to contact through PMs, that would be highly appreciated. Thanks!
Well, we know about it because the chroniclers write about it, the arab historians do as well, and then the crusaders coming through during the crusades speak about it.
Generally when you have multiple sources from different time periods that describe something, you can reasonably assume it existed. Theophanes in particular talks about it a couple of times, and given that he hated the regime that first employed it (Leo III) we have no reason to believe it wasn't real.