While it is confusing why Reddit wouldn't offer it as an olive branch, I don't think allowing Reddit Premium subscribers to access the API for free would be an ideal solution. It would be extremely confusing for the majority of users if they had to both 1) subscribe to Reddit Premium in order to use 3rd party apps at all, and then 2) also pay (likely via a second subscription) to use each specific 3rd party app. Both Reddit and the app developers would probably have to spend a significant amount of user support effort to explain why users need to pay twice (or more if they want to use more than one app) to access Reddit.

Imagine it from the perspective of a user who is new to Reddit and downloads one of those apps off the app store (possibly paying money to do so). After they fire it up they'll discover that they can't even use it until they subscribe to Reddit Premium. Due to app store policies the app can't offer the opportunity to subscribe from within the app, so the best they can do (on Android, at least, Apple doesn't even allow this) is to direct the user to sign up for an account and buy a premium subscription on Reddit itself first. Assume they get past this hurdle and then discover after a week or two that in order to continue using that app they have to sign up for a new, separate subscription through the app store so that the developer can get paid. I imagine a not insubstantial number of users would get extremely frustrated by all of this and take their frustration out on either the app developers or Reddit.