So what? Yes they both are in the Papeda group!
It's possible Yuzu could be its own species, with close relation to Ichang papeda, but there are several things that point in the other direction.
If they are so closely related to each other, like the DNA marker analysis seems to infer, what are the chances that the two would develop to be so different, with Yuzu taking a strong resemblance to mandarin? It is believed hybridization is the more likely possibility. It could even have its origins in an ambiguous zone between "natural species" and "artificial" hybridization. I don't think anyone is really 100 percent certain.
In terms of genetic marker studies, Yuzu usually groups closely to Ichangensis, but look at this study, they have Yuzu grouped in the mandarin group instead of Lemon/Lime/Ichangensis:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0166969 (Fig 2, but you can also see Fig 1)
(however, in all fairness, since a few different varieties in that mandarin cluster, like Kabosu and Sudachi, also have descent from Yuzu, it could have thrown off the matching algorithm and made Yuzu wrongly cluster with mandarins)