Later. In 1776 they were only concerned with their own freedom.
The Founders, 1776 people, weren’t keen on attempts to break up the republic they established. Jefferson, very much a 1776 person, didn’t think much of attempts by Aaron Burr to break off part of the country for Burr’s own rule.
Nobody wants to see the work they produced collapse before their eyes.
But they either believed in the principle of self determination, or they were just pretending to believe in it for political gain.
I like to think they really believed in the principle of self determination.
He has waged cruel war against human nature itself, violating its most sacred rights of life & liberty in the persons of a distant people who never offended him, captivating & carrying them into slavery in another hemisphere or to incur miserable death in their transportation thither.
That was taken out of the finished document. I don't think we know what the vote was. Jefferson says Georgia and South Carolina objected, but also blames some Northerners. But that Jefferson wrote it and most of the delegates don't seem to have objected indicates that many of the Founders were well aware of the contradiction between slavery and the "most sacred rights of life & liberty."
"The principle of self-determination" is an idea whose content and limits have to be worked out by people as new cases come up. If the Founders did not believe that enslaved people had the right to self-determination, then it's already clear that they believed that that principle had its limits.
But I don't think you're being serious. The Founders had one thing to worry about: winning independence from Britain. They weren't going to jeopardize that by pursuing other goals. But they based their desire for self-determination on principles, and the recognized that those principles could be applied to other situations. It's not about how we reinterpret them. It's about how many of them interpreted their own beliefs.