Specifically, the individual, commenting on a the flag story on a local news site, said:

Now, this particular joker was trying to say that there were lots of white people who “may as well have been slaves” in the North, or words to that effect. That is of course absurd because while there might have been (and later in history certainly was) a lot of exploitation of workers and poor and such, nobody was being owned, bred, treated like animals with the full force of the law, etc. Anyway, I found others making similar claims and claims and apparently some folks actually think that slavery was widespread in both the North and South.

Here are the data from the 1860 US Census, the census that measures the distribution of slaves at the start of the Civil War.

And I made a pie chart just for fun:

So now you know.