Yeah, running an encrypted bitstream through a lossy encoder like CELP is going to sound pretty awful (i.e., the content will be completely unrecoverable.) The whole idea behind a speech codec is to simplify the frequency-domain properties of the data in one way or another.

People are missing the real takeaway from the article, which is that VBR speech compression has serious vulnerabilities that CBR codecs won't share. That part wasn't obvious.