Considering Assange has probably been pretty careful and paranoid, I wouldn't be surprised if this was the strongest thing "they" could find.
But truthfully how hard would it be to honeytrap this guy? Suppose you go with my child pornography scenario, your sexy blonde Swedish secret agent only has to get close enough to slip a USB key into Assange's luggage so that when he flies through LHR customs can stop him and discover it. And if he checked any luggage it would be even easier.
One also assumes that he might be a little paranoid about sexy blonde Swedish secret agents hanging around his suitcase...
You'd expect that, wouldn't you. But then his whole career show less risk-aversion than most people. To put it mildly.