Amazon must believe there exists a much bigger market for SD and HD music, otherwise why bother. They also must believe this gives them some advantage over Apple and Spotify.
Amazon also sell a lot of electronics, HD sound could be used to drive sales.
Roon integration could offer Amazon a very valuable and luxury interface, a usp, especially when Apple and Spotify start to release comparable products.
This could move Roon from being a niche product to the mass market via Amazon. Added to which Roon in its current form and price must be reaching the zenith of its market?
The game is definitely over for Tidal, MQA was always a bit of a nonsense.