Wouldn’t some combination of unique session keys, PFS algorithms, or block encryption modes make this false? When would the headers encrypt to the same ciphertext unless you were using ECB mode with the same key?
I think you misinterpreted cortesoft's point, which I could try to make clearer:
"Even if every single bit of payload traffic was encrypted, a huge portion of the traffic actually sent over the wire would still be identical - TCP headers, for one thing, would share a lot of common bits for each packet."
