You are right of course, a 5.56 or .223 is far more deadly than a .45 pistol (I assume he is talking about a regular .45 auto). The .223 is more accurate but the .45 can be accurate also.
Depends at what range you are going to be shot at, no? At close range the .223 would more than likely blow right through you, whereas the .45 would take large parts of you with it if it didn't get stopped by bone or hard tissue. Farther out the .45 would lose a lot of its energy whereas the .223 would become deadlier as it began to tumble. .223 doesn't have the knock down power that larger rifle calibers have but when struck in a vital area the delivered energy is devastating.
Even FMJs will undergo explosive fragmentation inside 100 yards or so. But I agree with the general sentiment: only an idiot would prefer to be shot by a .223 over a .45. .223 wounds make .45 wounds look like a walk in the park. Very nasty.
Even FMJ bullet will yaw severly, about 180 degrees all told, depending on the thickness of the target along the path, and break into two major pieces, plus smaller fragments. If he/they had been using disintegrating bullets, there'd be no ballistic matches.