No more illegitimate than the claim by the Colonists that the British were "oppressing" them. The Canadians were under the exact same laws, but they didn't feel oppressed. The British Loyalists in the US also didn't feel oppressed.
So yeah, propaganda was spread to justify America's independence from the United Kingdom. Same stuff, different century.
The thirteen colonies lived under taxation without representation. Not so much with the Southern States in 1860. In fact, as I have shown, they actually controlled tarrif and taxation policy. They controlled the Supreme Court. In many of the years prior to the Civil War they controlled the White House.
The difference between 1830 and 1860 was that the Southern Democrats so feared Lincoln as an anti-slavery president that they refused to accept the results of the election.
But our Founders certainly did feel oppressed, for dozens of factual reasons listed in their Declaration.
Among those reasons, the Brits had already started, declared & waged war against them, and would hang them if captured.
Those are far from trivial or "at pleasure" reasons for "secession", they amounted to clear & present necessity and nothing remotely resembling those conditions in late 1860-'61.