The aero drag of wider tire is not a lot. It is more the wider tires are not slower. Wider tires allow: more air volume in tire, lower PSI. In turn those help ride quality.
The aero drag is the elephant in the room here. By this study’s measurements there wasn’t much difference, but what was the width of the rim they used with the narrow tyre?
A narrow tyre on a wide rim is a wide tyre. They don’t explain or address this issue. I suspect they would have used a wide rim with all the tyres, which is notionally a sensible thing to do, but in reality it’s completely flawed.
What are the results if you used a narrower rim with the narrower tyre so that the frontal area was actually reduced as much as it could have been?
Rene herse has been doing those tests since 2006 "back when go-fast tires were 23mm." They covered a lot of variety and their conclusion is the aerial drag is tiny and real world conditions are more important than the idealized condition of tests on a steel drum.
The tests in the reference were done with a 21mm inner rim width. I believe that is neither super wide nor narrow. Arguably a good test bed rim to isolate the difference in tire.
Though, wide rim equates to a wide tire I do not think is true (at least, unsupported). The deep dish rims are only aero if the width is close to that of the tire. Reducing the ratio of tire width to rim width is aero! If anything, a wider deep rim on a skinny tire is increasing are dynamics. The aero dynamics of a wheel is not just a function of the tire alone. A deeper dish of more equal width to the tire helps that teardrop shape you want for aerodynamics.
The reference does go into some more detail why wide rim is desirable to reduce drag. I won't paraphrase further here other than to conclude a wider rim would actually be favorable for the skinny tire.Regardless, a middle of the road rim width was used in that data sample.
A narrow tyre on a wide rim is a wide tyre. They don’t explain or address this issue. I suspect they would have used a wide rim with all the tyres, which is notionally a sensible thing to do, but in reality it’s completely flawed.
What are the results if you used a narrower rim with the narrower tyre so that the frontal area was actually reduced as much as it could have been?