Veeeeery interesting....
So basically, although you can't see what's being sent, you can fingerprint a known thing and look for that fingerprint. But you do need to know what it is you're looking for and have a copy of it. And this will work for e.g. nasties phoning home, but is no use for intercepting the content of actual communications. And would this work on encrypted packets going over a VPN?
Pretty neat. There's bound to be a downside, but on the whole it's mainly upside.