And what appears lost in these considerations, is question of what undercover LEO operatives were armed with that day, like before little mr. red boots got his hands on a rifle.
Even the battle-rattle outfitted LEO's who waded into the scene likely where carrying .40's or .45 acp's, in addition to M-4 sort of Stoner variant, although I'm not suggesting they used those instead of what possibly & most likely(?) were .223 Remington equivalent.
As you also noted, but not in the following so many words; the outwards appearance of M-4 look-alikes does not mean that those would absolutely be chambered 5.56 x 45 Nato, or else civilian near equivalency .223.
Those weapons can be chambered for a variety of rounds, including in that range, sub-range of frangibility.
Just .223 alone can be loaded with a variety of projectiles, which do not all share identical terminal ballistic performance characteristics when coupled with range of powder loading.
That leaves yet another small array of loose ends, of the sort which call for some examination, before one were to go too far in direction of assuming that their own initial assumptions should be safely taken as fact.
Those are not the only loose ends, yet are one of many that could be known of and identified, among potentially yet more than anyone here on this forum has openly pondered upon.
The very real attraction of the .300 Blackout round is that it is essentially a .30 caliber bullet in a swedged out.223 case, and requires very little modification of the arm besides replacement of the upper and, iirc, different magazines.
The rifle looks the same outwardly as the .223/5.56X45 variants, but packs a harder wallop with a subsonic round that can be more effectively sound suppressed than the 5.56 because of the lack of sonic 'crack' from the bullet.
The basic Stoner design has been manufactured in a number of calibers besides .223/5.56 X 45.