Of course not, the article states clearly that they were not killed by friendly fire, merely that the sniper has access to American made ammo, there’s no implication that there’s an American sniper shooting British soldiers.
Again, how can they tell it's american-made ammo from the projectile, when British forces are equipped with ammo that uses the very same projectile?
Does L2A1 have a green painted tip on the bullet? If not, that may be how they determined it was M855.