Its an interesting systematic error if it is one as they have spent a lot of time and effort looking for it. The real test will be if someone else can reproduce the effect , I suspect the answer to this one will not be quick or they would already have solved it. Probably a mistake somewhere though I agree
My bet would be on an error somewhere (whether calibration of time between sites, distance calculations at that moment, etc.).
Although it would be neat to see if someone would run the numbers on the gravatic lensing at this distance. It could be cool to discover that 'massless' particles are not subject to the gravatic lensing that would distort the measurements of c that could be made here in the Earth's and Sol's gravity well.
What they measured might actually be the 'accurate' c that is nearly impossible to measure using light and an instrument with any mass...