A single article in the liberal blogosphere does not an argument make. You’re making a positive claim that the reason for the Second Amendment, with no caveats or additional explanation, was to allow for slave patrols. As support a liberal writer cites a few slaveholders who argue for a Federal protection.
But history shows a different story.
So in Federalist 29 Alexander Hamilton laid out an explanation that obviously inspired the Second Amendment in part, and he was no slave holder but a northerner. James Madison, a Virginian protective of slavery, was a strong advocate of the Constitution before the Bill of Rights was ever discussed. Further, Madison’s original text of the amendment actually says country and not state, again Madison was a Virginia slaveholder and he was fine with the “free country” text since he wrote it. Further, Madison’s original text very unambiguously suggests it as an individual right. It seems highly unlikely to me that either James Madison or Alexander Hamilton were supporting this concept for the slave patrols. In fact, there is a strong legal argument to be made the original constitution in no way at all would have allowed the Federal government to interfere with the operation of State slave patrols.
Further, George Mason painted as only caring about it from a slavery perspective, authored the Virginia constitution’s arms protection before the constitution was written and it explicitly is drawn from the English Bill of Rights and advances it as a general concept of liberty from tyranny. We can argue that he was being duplicitous, but there was no reason for a Virginian to be duplicitous about slave patrols in 18th century Virginia.
So yes, some people were concerned about slave patrols, but it doesn’t appear that is the “reason” for the Second Amendment. It appears to have had support in both North and South and for more than just one reason. It’s also highly unlikely it was given to insure Virginia ratified the Bill of Rights, the argument might make more sense if it was promised so that Virginia would ratify the Constitution, but Virginia ratified the Constitution without any amendments so even that’s fairly weak. It’s highly unlikely that Virginia, home of Patrick Henry, wouldn’t have ratified the other amendments in the bill of rights that they considered extremely important just because it didn’t include the amendment concerning the right to bear arms.