Is Easy Sounds a subsidiary of Yamaha corp? It seems like a completely separate company and yet its sole existence is to promote Yamaha products. Strange to me

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.

yet its sole existence is to promote Yamaha products

https://easysounds.eshop.t-online.de/epages/Shop40690.sf/en_GB/?ObjectPath=/Shops/Shop40690/Categories /"Roland%20JP-8000"

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.