If you consider the flow reattachment as a horizontal cross section of the wheel viewed from the side, a deeper wheel will have more of its "height" that has sufficient depth to reattach for such a wide tire, and as you add up/integrate those aero gains there is probably some optimal depth that would help. IIRC the only published data I've seen for gravel widths is the swissside data that shows ~2-5w

https://www.swissside.com/en-us/blogs/news/gravel-report