It is pretty extreme but I can't say I'm exactly surprised, metallurgy has clearly advanced quite a bit since the days when 556 NATO pressures in chamber were standardized 50+ years ago. If you have DoD budget levels of money to spend on a barrel you can clearly do higher.
Barrels that meet 556 NATO spec are as cheap per unit as $100, a really nice one is $300. If it's not a problem that your barrel alone might cost $700 I can see this being totally possible.
The 6.8x51mm will not defeat any armor that M2AP won't. NIJ certified level IV armor will stop M2AP, 6.5 Creedmoor, .300 Win Mag, and in some cases, up to .338 Lapua Magnum. The last two are far more powerful, with greater velocity and sectional density, than 6.8x51mm, and they still won't reliably penetrate with a single hit.
Armor penetration was a stated goal of the Army's NGSW program, and the new cartridge will penetrate better than 5.56, but it's still not going to penetrate modern level IV armor.
in some of the DIY-drone-strike footage I've seen recently it appears that the russians are wearing no plates at all.
The new 6.8 is a much heavier and higher pressure round than the 556 and delivers far greater range and body armor defeat capability. The pressure is so great (80,000 psi, 551.6 MPa) that the case head (the 'back' part where firing pin hits) is made of stainless steel instead of brass; a 'composite' (steal + brass) case. I don't think that has ever been done in a military infantry weapon; all steel cases have been around a long time, but those are about cost saving as opposed to performance.
80,000 psi is pretty extreme in a rifle round. The high end of traditional brass case ammunition is 65,000 psi. That includes high performance rifle calibers like 338 Lapua Magnum and 22-250.
Also, the XM157 Fire Control optic being built for these new rifles is remarkable: "variable magnification optic (1X8), backup etched reticle, laser rangefinder, ballistic calculator, atmospheric sensor suite, compass, Intra-Soldier Wireless, visible and infrared aiming lasers, and a digital display overlay."
Due to the undoubtedly high cost these weapons will be limited to the 'first responder' combat forces and M4/M249 will be the mainstay for the rest for many years yet. But this commitment including the official designation of a new cartridge is a clear signal that the DOD is taking infantry weapon performance rather seriously.
[1] https://www.army.mil/article/255827/army_awards_next_generat... "The XM5 Rifle will replace the M4/M4A1 carbine within the close combat force, and the XM250 Automatic Rifle is the planned replacement for the M249 Squad Automatic Weapon"