Implicit deny == the browser decides this third party is not allowed to request storage access and immediately rejects. This could be the result of policy or a user facing feature à la "Don't ask me again."
Explicit deny == the user gets prompted and chooses "Don't allow."

WebKit/Safari has seen misuse of the Storage Access API where the caller measures the time for the document.requestStorageAccess() promise to resolve/reject and changes behavior based on whether it was implicit or explicit. The goal seems to be to pressure users to allow storage access if they get prompted. In the specific case, the tap to play a video both starts playback and calls document.requestStorageAccess(). If the user is prompted and explicitly denies storage access, the video stops. The user can clearly see that it's possible to watch the video without storage access but is punished for not opting in. We've received multiple reports of this.

This is a tricky issue because of timing. One way is to always delay the resolve/reject. Another is to hang rather than reject and only execute the promise completion handler on resolve. A third would be to offer the user to some way lie, along the lines of "tell them I said yes but actually block access." I'm not sure the third option is something we could explain to users.