Maybe they don’t have 100% of the rights to the hardware in order to open source it? I don’t think they made this hardware in house. They would have had to outsource it.
People underestimate how much work open sourcing something acrually is. Not trying to defend Spotify, fuck Spotify, but open sourcing something isn't free.
The code may contain some proprietary things they want to remove. Maybe it's not up to public standards and they don't want to be looked down upon. Maybe it could reveal vulnerabilities in other code if not cleaned up.
Plus knowing how most companies operates, there are all kinds of secrets, API key and others in the repo that needs to be thoroughly removed before releasing to the public.