The problem with this argument is that the American Revolution, if it was to save slavery would never have allowed democracy but would have created oligarchy (which it did to some extent in the action of the slaveholding elite present at the creation of the Constitution). But democracy appeared and the American Revolution emerged as a republic moving into democracy in a compromise with slavery.
We should consider that Britain was not beyond slavery in 1776 (?) and that they also were concerned rightly the new Americans would be genocidal to the Indians.

Again, to see the half-truth only in this argument: consider Athenian democracy: was it founded to preserve slavery? Hardly, there was no challenge to slavery but democracy emerged somewhat chaotically but on its own terms.

The University of South Carolina historian holds that the American Revolution was launched to defend the institution of slavery against the British Empire.

Source: Historian Woody Holton launches 1619 Project-inspired attack on the American Revolution - World Socialist Web Site