I have read that. There are actually quotes by Lincoln that support that view. Of course, Lincoln was on record as supporting secession too.
There was also a statement - made by Lincoln at Gettysburg - that implied the war was fought by the North for the purpose of equality. It was understood at the time Lincoln was referring not to equality between men and women, but equality between free men and slaves. This was controversial in the North and South.
And too, the North had imposed confiscatory taxation on imports which had a disparate impact on the South. For some reason the South didn't like confiscatory taxation.
To say Southern soldiers, some of which owned slaves, were fighting to preserve slavery is like saying U.S. soldiers in the Jim-Crow era army went ashore on D-Day “to preserve segregation.”
A clever analogy which might be valid if, for example, Hitler was known as an ardent anti-segregationists and was feared by US soldiers as someone who would force integration on them.
But such is not even remotely the case.
By stark contrast, Lincoln was thought an ardent abolitionist by Confederates and was feared by them as someone who would impose it on them, if possible.
And indeed, turns out their fears were 100% justified.