They don't need to know you have evidence. All they need to know is that you deliberately caused them to erase your device under the guise of unlocking it. Which means your device could have had evidence, and now you've made it impossible to prove that it didn't.
IANAL, but I don't think so. They don't have to prove that the data you just wiped contained evidence of wrongdoing, they just have to prove that you wiped the data after they asked for it. That's kind of the point of "destruction of evidence". If they already knew the contents of the data, they wouldn't actually need it, now would they?
