True enough. And there is an ancient history of slave soldiers, particularly in the Islamic world.
But a slave, by definition, is no more a volunteer than a horse is.
This is, of course, why slavery is very nearly the ultimate evil of which humans are capable.
The way to create a “myth” is to set up a strawman, attribute it to your opponent, and then knock it down. Gates gives the game away when he says: “No black rebel units ever fought Union forces, although many slaves fought alongside their owners...” Perhaps someone has claimed that there were black companies, brigades, etc., BUT I have never seen such a claim. What I have seen are claims that blacks fought on the Confederate side. So even Gates is admitting that many slaves fought alongside their owners against the invading Yankees. Moreover, I suspect that it is likely many men considered “black” today, but who were of mixed race, fought in the ranks.
The only myth in the story is the suggestion that slavery was the only or main “cause” for the support of the war.
Please note that the same group of leftists also have to explain away the “slave narratives” that were compiled during the 1930’s.
No one should mourn the end of slavery, but slavery and the War Between the States are far more complicated and interesting than the typical Northern “triumphalist narrative” that passes for history in the minds of most.
FOr liberals, wouldn’t that be a good thing that blacks were not segregated into their own units, like they were in the north (under white leaders of course, because the north wasn’t exactly the bastion of equality).