I think the basic misunderstanding is not so much Android vs iOS, but tablet vs phone.
The tablet app is meant to be a full representation of the desktop app and includes (almost) all functionality, including the alphabet picker.
The phone has a different paradigm: as the limited screen real estate makes it impossible to offer the full Roon experience, the phone remote is more of a casual remote, offering direct access to often-used entries for starting music quickly. I find that I tend to use discover far more on the Phone UI.
There’s no Edit functionality, no Focus and artist/album browsing is rudimentary. I can see where Roon is aiming at, although I agree that an alphabet slider is a necessity in the artists/albums views.
That said, I find myself liking the Phone UI immensely. Whenever I like to reach content that is not available in Recent/Discover/Bookmarks, I find that the search function in the Phone UI is quicker than any scrolling list could ever be.
I have used Sonos and iPeng before and found the miniaturised ‘right-hand alphabet list’ scrolling near useless. And I doubt that fast/accelerated scrolling solves anything with album lists going into the thousands.