It is.

If their app is open and full screen, search within the app first.

No it isn’t. If you have Google maps open and suddenly decide you want to go to McDonald’s, then sure, that voice command should go to Google Maps. But if you make a voice command to send an email to someone, it shouldn’t open Gmail instead of your default mail app just because Google maps is open.

Similarly, if you’re watching a video on YouTube and want to search for a cat video, sure, the voice command should search in YouTube. But if you want to listen to music and have Spotify set as your default music app, it shouldn’t send the request to YouTube Music just because YouTube is open.

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.

The distinction is that YouTube and YouTube music are different apps

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

I'm using a Chromecast with Google TV, and YouTube and YouTube Music are separate apps.

On Roku there is only 1 app.