Certainly isn’t limited to hornady cartridges. I’m fairly confident if you loaded up a bunch of 6 BR, 6 Dasher, 6.5x47, and creedmoors in a rail gun, the odds of the BR/Dasher/x47 outperforming the creedmoors is high.
I think the biggest thing about the new hornady cartridges is throat spec. Parallel freebore slightly larger than bullet diameter of adequate length to get the bullet out of the case might make up for some runout or other shortcomings. There’s a reason competitive shooters have spec’d custom reamers that way for a long time.
I think there is something to “inherently accurate” and maybe it’s just less particular about what you feed it but it’s definitely a thing. I’m of the opinion that 6.5x47 is a touch less finicky than creedmoor and 260 but that doesn’t mean there’s not a pile of people that’ll outshoot me and my x47s with creedmoors and 260s.