To understand why certain products, which are seemingly unrelated to Google's core competency, have stayed with Google and haven't moved on to Alphabet, or why Google created a product such as OnHub and not Nest, you have to realize why all of these product are under Google - to track all the data that goes through them under the same generic/unified "privacy" policy, and then feed it into its advertising system.
That's why OnHub is a Google product.