Do you understand the technical differences in the least? I can assure you I do, and as a practical matter for purposes of the discussion regarding Sandy Hook, 5.56 and .223 are interchangeable.
Minutia regarding chamber dimensions, brass thickness, and chamber pressures are not relevant issues here.
They aren’t interchangeable if your gun has only a .223 chamber.
If it is chambered for both then it can handle both.
I have read enough on this in the few minutes to see that if it is only chambered for .223 it can cause a problem.
THEY ARE DIFFERENT CASINGS.
And it is relevant if Vance said they were .223 casings.