Well, there were about 150K Jews in the U.S., 25K of which were in the Confederacy. I do not know how many Jews were in Delaware, Kentucky, Maryland, and Missouri, where slavery was still legal.
The confederacy’s 11 states had 316,632 slave owners out of a free population of 5,582,222. The same census puts them at about 3 million slaves.
The union had just shy of 640K slaves in the four aforementioned states in 1860. I do not know the sum Jewish population of these four states in 1860 but it was about 85K in 1899. It is POSSIBLE that 78% of UNION slaveowners were Jewish IF the average slave to slaveowner ratio were around 10 to 1, as per the phenomenon one sees in the south with the average slave owner having nearly 10 slaves AND god damned near every Jew in that state were a slaveowner, as there were clearly less than 85K Jews in those four states at that time. This falls apart because even someone with 30 slaves and a family of six, means the average member of that family would still be at 5* slaves per person, well under the average of 10 per slaveowner. Even if a family was a heavy owner of slaves, they are all considered the property of the patriarch and even if entire families share in the ownership of slaves, the records only ascribe them to one party, so even if every Jew had access to slaves, only a fraction of them would be categorized as "slave owners", again making it mathematically impossible.
The 1860 census data is available for anyone who wants to find the right data.
I do not doubt that Jews were overrepresented by a large margin in 1840-1860, when compared on a per capita basis, but the ridiculous stat being presented is not even within the realm of possibility. Even the steelmanned assessment that possibly 78% of Union slaveowners were Jewish does not pass muster. It is POSSIBLE that 78% of Union slaves were owned by a Jewish master, but I am not asserting that and you'll have to comb the records yourself. I am merely noting that I do not doubt overrepresentation, but the levels being asserted are just categorically impossible.