Just an observation about a possible difference (granted not the most practical one). Bought seed of xanthina and hugonis from sheffields seed back in early 2019. Would have been sown outside May/June 2019. While the xanthina seedlings have started flowering none of the hugonis seedlings have buds at this stage (seedlings of both are all similarly sized).

-edit 17 Sept 2021-
Now the seedlings are flowering more and in a more mature way compared to the first few half sized flowers.
So far they all appear to be single, some are lighter yellow, a few are as yellow as Golden Cheronese. The variance may be an indicator they aren't exactly what they claim to be, still something within the yellow diploid pimpinellifoliae section.
The hugonis seedlings are still without flower buds.

I got this response from A.C Tunningley who is now growing the specimen pictured here in my garden as R. xanthina. I received it from Sequoia Nursery. He is growing it along side his specimen of R. hugonis received from Forest Farms.

"This morning I checked on the hips from your probable xanthina specimen. They are clearly different from my hugonis. There is no chance that these are the same plants. I bet what you sent me is the typical xanthina."

"the two plants are similar enough to pass for one another in many ways although hugonis has a much more saturated color for me and a far superior growth habit."

Rosa xanthina was sometimes called Double hugonis in the years soon after its introduction. See References under Rosa xanthina. Whether roses sold under the name of Double hugonis/Rosa hugonis flore plena/Rosa hugonis floro pleno in these times are Rosa xanthina or something else is not clear.