Sure, it was.
My point is that claiming something like "no European state faced an existential threat from Arabs" is a bit circular -- the definition of Europe is very close to being those parts of the Roman world which were not (permanently) conquered. (Or if you think it's a geographic term, then the reason this particular geographic lumping is important, as opposed to say Mediterranean.)
Well, it [Constantinople and Byzantium] certainly was a European state, the Eastern Roman Empire, which kept the lights on when as the Western one was dissolving and finding itself again after Rome declined.