That is another lie.
The first large-scale emancipation occurred 1782-1790. It didn’t take a constitutional amendment or federal intervention by a proclamation or a law. Each slave was manumitted by his or her owner without coercion. It was the result of promises to revolutionary veterans, the US Constitution, personal ethics, and state laws.
In 1776, the population of the new United States of America was 2.5 million. More than 500,000 blacks were part of that population, including about 450,000 slaves.
In 1782, Virginia led the charge with the 1782 Manumission Act, which allowed owners to free slaves by will or deed. In contrast, Pennsylvania, Connecticut, New York, and New Jersey implemented gradual emancipation. Notice that each state chose its own course.
Gradual emancipation allowed the development of economic knowledge and work skill mastery that would enable the slave to make a decent living without oversight. Thus, for owners taking advantage of this, they would continue to make income from the slave in the meantime and allow owners to adjust economic realities to a free workforce.
The wave of emancipation ended in 1790. In that census, there were 59,000 free blacks in the United States. Of that, Virginia freed at least 20,000.