No1 suggested it was "as widespread" - just that it existed. The point of the article was that NY was just as guilty as having slavery as any, and I guarantee you the impression we as kids got was that slavery was only a southern thing.
Conveniently by 1850, most "northern" states had banned slavery, so your point is moot.
As for it being as widespread - no, for 2 basic reasons I think:
a) the originating colonists' communities were inherently different in population density (= small land grants vs large) and attitude;
b) almost everything north of the M-D line is ROCK. There is hardly any good farming for the taking to make it a serious business North.
Basically, no big-business farms = fewer slaves.
But it doesn't erase the fact that northerners had slaves, something I guarantee we were not ever told in high school even.
Another fact that is conveniently forgotten is that there were very few slaves in the north by 1800. Up until 1776 - any slaves that were owned were owned by British colonists. There WAS no USA until AFTER the revolution.
The fact remains that most of the slaves were in the South. Whatever reasons for that that Southerners may come up with does not change the facts.
Saying that the only reason the North didn't have many slaves is that the soil in the North is all rocky is not true. Ohio, Michigan and Indiana have much better farmland than the Southern states. There have been settlers in Michigan since the 1600s - but there were very few slaves in Michigan. There were many mills and factories in the North - it would have been cheaper for the mill owners to use slaves rather than employees - but they didn't.
The reason they didn't is because most Northerns thought that slavery was wrong and wanted no part of it - not because the soil was rocky.
Certainly you'll find some slaves in the North if you go back far enough. You'll find slaves in London, Paris and Rome if you go back far enough. But you didn't find them in the 1850s and 1860s like you did in the American South.
The "you guys did it too" argument just doesn hold water.