Looking at the final string of comments on the previous thread, I suspect they are doing it because WebP has finally reached enough critical mass, in particular

Looking at the final string of comments on the previous thread, I suspect they are doing it because WebP has finally reached enough critical mass, in particular with some CDNs and mod_pagespeed (plus some user agent or accept header confusion if I recall), for continuing not to implement WebP simply making too little sense. Better late than never...