He didn’t, malware guys use UPX and it’s true that antiviruses scream bloody murder when they see it. It’s also true you can’t see what’s inside unless you have special tools to do so. UPX also has one huge downside, it’s its RAM usage, due to it’s inner workings it’s unable to use optimisations that normal binaries can like page sharing.
http://phrack.org/issues/71/6.html#article here’s the original paper for the intrested.