Google isn't asking Roku to open a separate app, they're asking the search to be performed in the open app first.

What the person above you originally said.

But YouTube is a music streaming platform. The most popular, in fact (no, not YouTube music). I constantly listen to music across both Spotify and YouTube. YouTube serves video and doesn't present music in the way we usually think of it, albums sorted by artist, chronologically presented... But that isn't really how the younger gen listens to music. It is a music app and a common way a lot of people consume their music.

They're not on TVs. YTM is a section inside YT app on the TV.