From your link:
“It is not at all surprising, such being the character of the Government of the United States, that it should assume to possess power over all the institutions of the country. The agitations on the subject of slavery, are the natural results of the consolidation of the Government. Responsibility, follows power; and if the people of the North, have the power by Congress—”to promote the general welfare of the United States,” by any means they deem expedient—why should they not assail and overthrow the institution of slavery in the South?”
Kind of undercuts the false premise that there was nothing to fear about the future of slavery, doesn’t it?
This is like people finding those secession statements from three states, and ignoring Virginia's secession statement, which contradicts what they want to show.
Did you bother to read the economic portion of the statement?