Isn't alternate history revisionist by definition?
No, it is not.
Alternate history does not necessarily have some ideological goal, unlike historical revisionism. Alternate history writings may include some of the same goals as revisionist history- but I imagine such TLs are usually terrible (see right-wing FH, for example, such as
Caliphate and the like.)
As to robertp's expertise, he certainly does have a good idea of the military realities of the Confederacy. But he makes the Confederacy out to be a far nicer place than it actually was. I again recommend the farce The Black and the Grey, which I found shortly after coming to this site (and I lurked well before March, thank you.)
As to his "evil Confederacy" TL, it read to me like a parody- a reductio ad absurdam targeted at those who criticised his vision of the Confederacy. An evil Confederacy is thus, as implausible as the rest of the events in that TL, where the Confederacy starts on the path of some Draka-analogue. It is not a serious consideration of the flaws of the Confederacy.
Interestingly to the discussion, there were some (admittedly short) discussions at killing the slave trade clause in the CS Constitution. The main reason was to allow a limited slave trade with Cuba, who at that time still legally possessed slaves. Obviously, it didn't get killed but opening the slave blocks of Havana could easily be a goal in a post-ACW Confederacy's expansion. Do not forget that Cuba was the second to last in the Western Hemisphere to legally abolish slavery.
EDIT: Also, if you need some evidence of Southern leader's support for a "Golden Circle" beyond the simple maintenance of the slave state-free state balance, I would suggest you look into much of the enthusiasm surrounding the Spanish-American War. The conquest of Cuba is often regarded as a factor which helped to re-unite the North and South- and the expansion of Southern agricultural business there was certainly a factor. Cuba had become by that time, a Megali Ideal sort of national destiny, slavery or not.