Actually RevenueCat is requiring the App Store Connect API key for some of their functionality (e.g., as mentioned at https://www.revenuecat.com/docs/service-credentials/itunesconnect-app-specific-shared-secret/app-store-connect-api-key-configuration).
Thus I still have the same question as Blinker73: is one's own RevenueCat account considered a 3rd party, and is it thus allowed and/or safe to give RevenueCat a App Store Connect API key?