No, Easy Sounds is not a subsidiary of Yamaha. Easy Sounds is a separate company who does a lot of sound programming for Yamaha Synthesizers, but they also do other work providing content for Ableton, NI and others. When you see a company that makes cases for the iPhone, do you assume that they are owned by Apple as well? I would assume that they are not and simply a company that has a business selling accessories for another larger company. It should be noted that the Music Production Guide is created by a partnership between Easy Sounds (who hosts it) and Cool Webinars, a service provider who does training and consulting on Yamaha Synthesizers primarily.
This page, among others, seems to indicate Yamaha products are not their only focus.
It's fairly common industry practice in other platform+content domains for some 3rd party vendor to provide online, in person, or in-print training services and materials as it helps establish the said 3rd party content provider as an authority and is a source of free advertising. It's smart business.