As surprising as this may seem, there is some truth in these allegations, but of course this does not change in any way their antisemitic character.
Why there is some truth
The French Triangular Trade was conducted primarily from the harbor of Nantes, from where departed almost as many slave-carrying ships as from all the other French harbors combined. Nevertheless, hundred of slave ships departed from the harbor of Bordeaux, deporting in the 1750-1850 an estimate of 130 000 African slaves (mostly to the French Antilles). The slave trade, or more accurately the global trade with the French Antilles in which it took place, contributed in a significant way to the prosperity of the city Bordeaux.
As it turned out, Bordeaux was also where a significant number of Jews expelled from Castile and Aragon by the Edict of Expulsion of 1492 had found refuge, where they became known as Nouveaux chrétiens (new christians) or Chrétiens portugais (Portuguese christians) because most of them had transited through Portugal before being expelled again in 1497 and because most of them had pretended to have converted to save their lives. For two and half centuries, they were subjected to the usual discriminations inflicted on Jews in Western Europe at the time, but they nevertheless prospered, contributing significantly to the economic and medical development of the region. Finally, in 1723, they gained (of course moyennant finance, as we say in France) official recognition of their citizenship and the right to practice their religion.
Among this Jewish community was a family called the Gradis, whose members were Atlantic traders, specialized in sugar and trade with Canada. They became extremely prosperous in the first half of the XVIIIth century, gaining a quasi-monopoly of French trade with numerous colonial and quasi-colonial French possessions in Africa and the Americas. As their preeminent status as Atlantic traders in the relevant period, it is thus true that this family organized a significant part of and profited from the slave trade out of Bordeaux; as it organized a significant part of the trade out of Bordeaux more generally. Because their continued prosperity and high-standing during the XVIIIth and XIXth century, several members of this family reached official positions in the French state, and several of them also occupied various leadership positions among the French jewish community.
Based on this family alone (and actually many more had comparable though less successful trajectories), there is thus some truth in the assertion "The French slave trade was organized and conducted by official leaders of the jewish French community."
Why even a rapid examination of the facts show that presenting it in this way is antisemitic
Most of the French slave trade was conducted from Nantes, which was entirely devoid of a jewish presence. Singling out jewish involvement is therefore dishonest. Slave trade was always a small part of the Gradis business: nothing suggests that they were proportionally more involved in the slave trade than any other Atlantic trader of the time (in France at least, there never existed dedicated slave ships: slave trade was considered a part of normal trade and conducted by the same crews with the same ships). Besides, even a quick perusal of the relevant facts establish quite clearly that the involvement of preeminent members of the French jewish community was a side-effect of several contingent facts: 1) the presence of a jewish community in the Bordeaux region, itself a legacy of much persecution and discrimination 2) the success of this city in Atlantic trade 3) the peculiar success of a handful families in Atlantic trade in precisely the period in which Bordeaux was involved in slave trade.
And what can I say to people who support such claims ?
You can tell them that an honest assessment of the historical facts is: "After centuries of persecution and discrimination, a jewish community became prosperous in the Bordeaux area, especially by investing in Atlantic trade. Preeminent members of this community came to a dominant position in Atlantic trade out of Bordeaux from 1730 to the end of the XIXth century. They participated in all aspects of this trade, including slave trade in the period 1750-1815. They were never the main slave traders in France (these were in Nantes) nor is there any suggestion that their religion played any role in their involvement."
History of the jewish community of Bordeaux:
Slave trade from Bordeaux:
Portuguese Jews in the Bordeaux area