Even if fuel cells contain combustion, such an engine isn't called an "internal combustion engine". Conventional meanings apply to longer phrases, not just individual words. (I'm not sure on the exact definition of "combustion", but the top Google result is "rapid chemical combination of a substance with oxygen, involving the production of heat and light." I doubt the temperature of a fuel cell reaction zone gets high enough for more than a rare photon of visible light. The temperature in a gasoline or diesel cylinder definitely does.)