It is a common technique he uses. Rather than just say dumb shit - or straight-ahead lie - he says that people "told" him something. Therefore he has deniability. Does it all all the time in his speeches.
This is particularly stupid. Why would anarchists all take the the same plane? Why would they all dress as conspicuously as possible - and bring their weaponry somehow with them?
We are to expect that they did this and NO ONE reported it? Only one person - who happened to be close to the president - witnessed it? No one had a cell phone? The airline did nothing?
This is maybe his dumbest lie.