Reminds me of this Tom Clancy like book I read back in the 1980's where the US used a Fuel Air Explosive against the fictional enemy and much was made in the fictional press about it by linking the weapon to the word nuclear, even though it was not a nuclear explosion.
It's a nasty weapon for sure, and should not be used anywhere near any civilian settlements (towns, cities, villages) but it is not by definition a WMD.
Needless to say if there are considerable civilian casualties, Trump will lose his leverage on going after Assad.