Since radon is an intert gas, it has a pretty small atomic diameter. This means it goes through most filter media. In fact, if you had something which trapped radon one side of it, it would probably require several tens of bar in pressure difference in order to be effective. In fact, consider the size of an N2 and O2 molecule, then compare with a radon atom....case closed!
The BQM^-3 rely on radon daughters being charged and sticking to a substrate, I suspect by having a porous compound which likes charged stuff. Then when you get your Bq reading, you correlate this with how much gas you have pulled through it. You assume there has been no significant decay between when you finished the test and all the stuff didn't get through. I imagine, like most radiation related things, it's a bit of a statistical fudge.