That doesn’t fit the facts.
There are numerous written documents referring to northern states as being much more racist. Here are some examples:
1818-1822: South Carolina Senator Robert Young Hayne described his personal observation of the fate of Southern blacks who fled north: “…there does not exist on the face of the whole earth, a population so poor, so retched, so vile, so loathsome, so utterly destitute of all the comforts, conveniences, and decencies of life, as the unfortunate blacks of Philadelphia, and New York and Boston.”
1831: French aristocrat Alexis de Tocqueville wrote, “Whosoever has inhabited the United States must have perceived that in those parts of the Union in which the negroes are no longer slaves, they have in no wise drawn nearer to whites. On the contrary, the prejudice of the race appears to be stronger in the States which have abolished slavery than in those where it still exists; and nowhere is it so intolerant as in those States where servitude has never been known.”
1837-1841: English writer James Silk Buckingham toured the United States and Canada, visiting all but two states. He wrote, “the prejudice of colour is not nearly so strong in the South as in the North.”
1841: English Quaker Joseph Sturge wrote, “In the course of conversation, the Governor [of Illinois] spoke of the prejudice against colour prevailing here than in the slave States. I may add, from my own observation, and much concurring testimony, that Philadelphia appears to be the metropolis of this odious prejudice, and that there is probably no city in the known world, where dislike, amounting to hatred of the coloured population, prevails more than in the city of brotherly love!”
Early 1853: Connecticut landscape architect Frederick Law Olmsted wrote, “The railroad company advertise to take colored people in second class but servants seem to go with their masters everywhere. Once, to-day, seeing a lady entering the car at a way-station…I offered my seat which had several vacancies round it. She accepted it, without thanking me and immediately installed in it a stout negro woman; took the adjoining seat herself, and seated the rest of her party before her…They all talked and laughed together; the girls munched confectionary out of the same paper, with a familiarity and closeness of intimacy that would have been noticed with astonishment, if not with manifest displeasure, in almost any chance company at the North.”
1862: Massachusetts justice of the peace John S. Rock wrote, “We are colonized in Boston. It is five times as difficult to get a house in a good location in Boston as it is in Philadelphia, and it is ten times more difficult for a colored mechanic to get employment than in Charleston.”
1862: English writer Edward Dicey wrote, “I hardly ever remember seeing a black employed as a shopman, or placed in any post of responsibility. As a rule, the blacks you meet in the Free States are shabbily, if not squalidly dressed; and, as far as I could learn, the instances of black men having made money by trade in the North, are very few in number.”
There you have opinions stated by black and white, north and south, foreigner and citizen. They all say the same thing: northern states were decidedly more racist than southern ones.