More misunderstanding by BIBA. It would be very shocking if the mitochondrial DNA wasn't mainly ligustica as that was the original cross, Mitochondrial DNA is mainly female and is passed on from mother to daughter at each crossing.... with little of it ever being attributed to males (or in this case drones). It is a historical record of where they started from, not where they are now.
The authors of the paper concluded ". A phylogenetic analysis, suggested that the matriline ‘Buckfast bee’ has remained most closely related to the A. mellifera ligustica race from which it originated in 1917, despite being cross-bred with many other A. mellifera races over the past 100 years".
https://tinyurl.com/y6m4rw85 free to download and read.