Even if they did, EVERY single founding father in the federal and state conventions that agreed to ratification - signed off on slavery as being legal.
Every single one.
Which IMHO is the Red-Diaper Revisionists' point. They're using an interpretation of Lincoln's revolution to attack the Framers.
Which they'd have to do, to lay the groundwork for an eventual Marxist revolution in the States, the way Lenin and Iskra had to attack first the Imperial Russian government, and then Kerensky's.
Just a simple statement of fact. No judgment, no condemnation, no criticism expressed or implied. Just the incontrovertible truth that each of these men was a slave owner, a fact supported by incontrovertible evidence. But there is one among you who continues to deny these facts and other facts when presented to him. That was the individual the statement was addressed to.