Just a SWAG, but I'm going to guess the reason the rounds are "optimized" for shorter barrels is due more to case capacity relative to bore diameter. In the cases you've listed the cartridges are meant to increase the effectiveness of a short barreled AR carbine. The effectiveness of the 6.8SPC and .300BLK depend less on raw bullet speed like the 5.56 by substituting bullet mass and diameter.
Now the other side is, picking on the .300BLK specifically, it will never be able to do with a 7.62x51/.308 cartridge firing platform will. No matter the barrel length, the 7.62 has more muscle and can generate more velocity. The .300BLK just lacks the case capacity.
My opinion when people say a round "isn't optimized" for a short barrel has more to do with the perceived "significant" velocity loss and increased blast and noise of firing, say, a .300 Win Mag from an 18" barrel versus a .308. This has nothing to do with a short-barreled .300 Mag being "no more powerful than a .308 with a lot more noise." (To grossly butcher the oft-cited claim about short-barreled .357 Magnums vs. 38 Specials.)