So the linked "sources" cite the 1860 U.S. census, MyJewishLearning, and JewishVirtualLibrary. I decided to ignore the latter two as I'm fairly confident they don't back up this claim. So out of curiosity I decided to check out the 1860 census and see what it said
It's really fascinating stuff, and I recommend anyone interested in history checking it out. You can read up on it here, and a separate page on statistics here
So what kind of information was taken in the census? Well, there were two "schedules": Schedule 1 was for free inhabitants, and Schedule 2 was for slaves. There were 14 questions on Schedule 1, including names of family members, age, race, sex, etc. There were no questions regarding a person's confessed religion
"But Ash!" I hear you cry. "Judaism isn't just a religion, and back then would have been classified as a race as well!" Which is true, but "Hebrew/Israelite/Jew" wasn't one of the options given. The races available to choose from were white, black, and mulatto. The state-by-state populations are organized into white, free colored, Indian, and slave
That's not to say Jews were left out entirely. One of the statistics pages includes a count of all churches. New York had the highest number of Jewish "churches" with 20, compared to, for example, 1,683 Methodist churches.
In fact, we can count the number of Jewish churches in all slave states in 1860 and come up with a number of..... 19 (out of 77, or 14.6%). There were fewer synagogues in the entire South than there were just in New York
"But Ash!" you cry yet again. "What if Jews were counted as foreign nationals?" Well, if we were, there would be no way to know, as nothing indicating Jews was on the list
There's also the question as to how to count the slave-owners, as that was something recorded on the individual schedules, but not included in the population reports. There are lists of occupations, although sadly "slave-owner" isn't one of them (but there were in Louisiana 3 "artifical flower makers" and 3 "bird fanciers" so that's cool). In order to determine how many Jews owned slaves, you would have to go through the individual schedules of all slave owners and... I suppose guess? which ones were Jewish?
(there are many resources which debunk the myth of Jews dominating the slave trade so I'm not going to go into the myth itself, but I just wanted to look at the census specifically since it's an odd thing to cite)