The 5.56 has a longer throat to run higher pressured ammo safely
This isn't quite correct. It's like saying the 30-06 has greater case capacity to run higher pressured ammo safely. The reality is, the longer leade allows more powder to be used to increase velocity without increasing pressure.
223 spec ammo is not loaded to lower pressure. It's loaded to make full pressure in a 223 spec chamber. Pressure is measured differently for the SAAMI 223 than it is for the NATO 5.56 and do not directly compare.
Another thing to remember, just as all commercial 223 is not loaded to the same pressure, neither is all milspec 5.56
