You can make lots of ammunition ‘caseless,' and the article only cited a caliber (6.8mm), not a specific cartridge (i.e. 6.8 SPC). Assuming it _is the 6.8spc, though, that ammunition has normally been of conventional construction.
I struggle to understand how a relatively stubby (short, low-SD) .27-cal bullet moving considerably slower than a .223/5.56 bullet is thought to penetrate armor better than the .223/5.56 itself. If bullet construction is significantly different, that's fine but then one could as easily adopt alternate bullet construction for the 5.56.
A larger-diameter bullet makes bigger holes in soft targets, and so is likely to have better ‘knock-down' than a .22-cal on _un_armored personnel, but somebody's gonna have to show me the data before I buy into the notion of superior armor penetration.