You can call it whatever you want - I call it pre-war propaganda."
So let's start here: the Confederacy formally declared war against the United States on May 6, 1861.
"Today" is June 6, so that is hardly "pre-war".
Second, this Northern editorial addressed only one of the South's arguments -- the one from "rebel leaders" which claimed "Northern hatred to the South".
The editorial argues, entirely correctly, that claim is, as you say here, a myth and a lie.
It was a lie in 1861 and still is in 2021.
Third, back to the definition of your word "propaganda".
So it's not hard to find dictionary definitions which refer to normal commercial advertising as "propaganda", thus rendering the term harmless & meaningless.
But the truth is that in political discourse propaganda is always understood to mean lies, deceptions, misdirections & biases.
I don't see any of those in this particular editorial.
enumerated: "The South’s claim of anti-South hatred was a legitimate reaction to hit pieces like this one, where Northern “editors” were attempting to turn Northern opinion against the South, leading up to the war."
Except, first, that you've reversed the sequence -- this editorial is in response to "rebel leaders" claims of "Northern hatred to the South", not the other way around.
Second of course, that "today" we're not "leading up to the war", but rather Confederates have already provoked war, started war, formally declared war and begun to wage war against the United States.
Under such circumstances that Northern editorial was entirely appropriate and unusually kind to average Southern citizens.
In case you missed them, here is a listing of Civil War engagements "so far", meaning through June 6, 1861.
There have been eight, producing about 150 casualties (plus an unknown number of Baltimore civilians wounded), of which killed total around 60:
| Date | Engagement | Military Units | Losses | Victor |
|---|---|---|---|---|
| April 12-14 | Fort Sumter, SC | Confederate artillery, Union garrison | None | CSA |
| April 15 | Evacuation of Fort Sumter, SC | Union garrison | Two Union soldiers killed, four wounded by accidental explosion | N.A. |
| April 19 | Baltimore Riots, MD | MA 6th, PA 26th vs secessionist crowd | 4 Union soldiers killed, 12 civilians killed, hundreds wounded | USA |
| May 10 | St. Louis Riots, MO | Union forces vs secessionist crowd | 4 Union soldiers killed, 3 prisoners, 28 civilians killed | USA |
| May 18-19 | Sewell's Point, VA | Union naval squadron vs Confederate shore artillery | 10 total | inconclusive |
| May 29- June 1 | Aquila Creek, VA | Union naval squadron vs Confederate shore artillery | 10 total | inconclusive |
| June 1 | Fairfax Court House, VA | detachments from CSA & USA armies | 8 on each side, 1 each killed | inconclusive |
| June 3 | Philippi, WVA | Union Dept of Ohio (McClellan), CSA infantry | Union 4, CSA 26 (killed or wounded) | USA |