Voice Therapy Efficacy hardly exists

So I checked my email this morning and had the email that the image above is from:

Subject: Voice Therapy Resources -- Forthcoming and Bestselling

Just so you know before you buy anything, just type "efficacy" and "voice" in Google. Guess how many efficacious voice therapies there are? 1! UNO! ONE! ...and it is for persons with Parkinson's disease! So does that mean one shouldn't trust a vocologist and not get voice therapy? No, I am just saying, why should I buy a new book about techniques that have been out for years but have not been shown yet to be Efficacious?