Anyone that wants to revoke the access this (or any previously used) app had to your spotify account, you can revoke access here: https://www.spotify.com/uk/account/apps/
Yeah, the absolute lack of any privacy statements makes me wary. They get access to my entire music history and personal info, and just say that they won't post anything? Pass.
To be clear, it looks like the oauth scope they're requesting only allows read-only access to your account. So it's not like they could actually post on your behalf without additional express permissions, barring a Spotify API bug (as far as I understand).
That said of course I agree they should have an explicit privacy policy regardless.