I am content to regard Yuzu as most likely having arisen from hybridization between Ichang papeda and sour mandarin or mandarin, probably not from a single hybridization event.
There may have been a cross between the two, and after that the offspring got propagated from seed, so there was further genetic mix-up.
In the initial cross, Ichang papeda probably would have been the female parent. The genetic marker studies show that Yuzu shares a much closer affinity to Ichang papeda than to mandarin.
"Sour mandarin" may be a sub-species of mandarin, kind of like a wild mandarin type, originally native to China.
Or it may be possible "sour mandarin" actually came from the regular species of mandarin but just with a little bit of Ichang papeda-type genes mixed in.
(If you look at the
Hybrid Origins of Citrus Varieties Inferred from DNA Marker Analysis of Nuclear and Organelle Genomes, Tokurou Shimizu, article,
Citrus sunki seems to stand out as the only near pure mandarin with some additional few genes like C. ichangensis or C. medica, probably the former)
If that is the case, then it may be of little meaning whether the ancestry came from sour mandarin or mandarin. And it is also of course a possibility sour mandarin may have ultimately arisen from Yuzu, rather than the other way around. Or even no direct parental relationship.
None of this is fact, it is informed speculation, an attempt to try to fit together the pieces.
We don't know if this hybridization resulted naturally, or came about as a result of human activity.
I will quote a source here:
" Swingle and Reece (1967) noted that: […] hybrids that show astonishing similarity to the Yuzu have now been produced in this country between the Ichang papeda and the satsuma orange (a form of C. reticulata ). "
https://idtools.org/id/citrus/citrusid/factsheet.php?name=YuzuWhich lends some support to the hybridization theory.