I understand that, but in four years, they couldn't get any physical proof?
Easily. If he’s talking about stuff he did last month, last year, in a foreign country, there’s no evidence to be had, it’s gone, inaccessible or meaningless. Usually what you need is for them to talk about the future in enough detail you can set surveillance and a sting, or they give away where evidence (like the videos they found) are stored. Sometimes you get enough of a detectable pattern from them talking about the past you can predict the future (if every time he goes to city X he talks about what’s available at a location they can narrow down then they’ve got him). But most of what recording people is about is waiting for that piece of information that let’s them get physical evidence, and then the tape is really only useful to get a warrant and help establish an ownership chain.