You do realize though those flechettes are much smaller in diameter, right? Combine how thin the projectile is with the speed and you've got a cartridge with as much penetration as a 5.56 at least IMO. Flechette diameter is 1.5 mm compared to 5.56 mm.
Smaller diameter certainly helps, but at 1/8 the weight and roughly 2/3 the velocity (or lower) the penetration capability just isn't there. If you believe a flechette can match 5.56mm for penetrating power and damage you have a vivid imagination. There's a reason why no military has developed a .06 caliber rifle to defeat body armor.
A simple muzzle energy calculation shows a single flechette leaves the barrel with a paltry 66 ft. lbs. of energy - less than .22LR at 135 ft. lbs. and way less than 5.56mm M-825 at 2700 FPS which has 1,004 ft. lbs. Muzzle energy isn't a strong indicator of terminal performance, but at a measly 6% of the energy of 5.56mm it is another strike against a flechette.
Evidence I can find of people actually trying this says the same thing. 12 gauge flechette rounds either fail or struggle to penetrate light body armor:
(This one is a bit weird, because the narrator implies flechettes penetrate soft armor well, yet the video clearly shows them failing to fully penetrate a piece of cardboard behind the armor. Painful, but certainly not lethal, in line with what the other videos show.)
Ill be fair here, flechettes are quite weird but theyre also very particular. The first video shows him clearly unable to actually load the things in properly and then seeing it not work properly as a result. I feel flechettes are very particular in how they must be shot to achieve maximum performance and efficency, loading in 30 flechettes into a shell and half of them backwards isnt the way to shoot them.