we got a request from a possible client who asked us if we could create a customized (branding) version of the iOS ownCloud Client. Can the version from the github repository be used to create a custom-branded version of the iOS-Client and to offer this version through the Apple App store? This https://owncloud.org/contribute/iOS-license-exception/ reads like it's not possible?
If it's not possible: Why is it possible for ownCloud Enterprise - customers? Because that version of the iOS client has a different license (commercial license)?
Correct. The GPL is not compatible with the Apple Store at this time. Enterprise customers are getting a relicensing under a commercial license. You or the client should contact sales@owncloud.com.
Nextcloud provides an iOS client which can be branded. Current versions of OC and NC are still very similar that they work with the other product as well.