I am sure of this. I help lead search for Hulu and Disney+ and own a RokuTV. Roku already allows you the option to voice search inside the app you are already in (this has to be enabled by the Roku app developer I believe).

This definitely refers to the universal search Roku offers, and, if true, is an absurd ask on Google's part.

From the Roku blog post:

First, Google continues to interfere with Roku’s independent search results, requiring that we preference YouTube over other content providers.

That's Roku's own search app Google are trying to influence.

https://www.roku.com/blog/en-gb/update-on-youtube-tv

I have a Roku. YouTube's app, along with all others I have used, only search content within that app.