I've not checked the link, but Louisiana's Jewish population was indeed under-recorded by the U.S. Census in 1860.
Elliot Ashkenazi in his work, The Business of Jews in Louisiana, brings out records from the predecessor of today's Dunn an Bradstreet Company. Dunn and Company in the 19th century were keeping confidential records on about anyone involved in the credit system of the day.
The records show Jewish families in about every trading town along Louisiana's rivers, far more people than the census would suggest.