>Sending an email is a different interaction than doing voice search.
Going from watching videos to playing music is a different interaction than doing voice search.
>Sending an email is an explicit intent - open whatever app I use to send emails.
Using a global voice commands (not search) has explicit behavior - use whatever app I have set to default for the functionality I am requesting. "Play Stairway to Heaven" should use my default music app. Note that "voice commands" is different from "voice search" in this context and is the alleged problem.
>Searching is an open query - find the most relevant results.
If I use Spotify as my default music app it is because I trust their music search more than YouTube Music. Otherwise YouTube Music would be my default music app.
There is also a massive contextual difference between a global Voice Search (using the Voice search icon on the Roku remote: it searches Roku) and using the Speech to Text option that may appear when already searching within a search field (which uses the search field of the app itself, in this case: Youtube)
>The separation between Youtube and Youtube Music is a technical minutia, they're both Youtube just different apps.
Google deciding there is a difference between the two means that there is a difference between the two for both a marketing perspective and whatever minor technical differences there are. If there were no differences there would not be a YouTube Music app and to pretend otherwise is disingenuous.