I didn't say it wouldn't fit or that one couldn't put the parts together and shoot it. I said it was "not so good an idea." There are a lot of dumb combinations of cartridges, barrels, and firearms that people may get away with shooting, but are not a good idea for a variety of reasons.
I have shot 10mm with a 10mm barrel in a Glock 21, just to see if it would "work." I don't recommend it.
Heck, once somebody shot a whole magazine of 9mm through one of my .357 SIG Glocks when I wasn't looking. I don't recommend that either.
By the same logic one "can" shoot .40SW through many 10mm pistols but it's not a good idea either.
And for this case, since I have both guns sitting here, here is the difference:
Glock 21 with factory .45 ACP barrel
Glock 21 with factory Glock 20 10mm barrel:
You can see there's roughly 3x the clearance on each side of the barrel where it meets the breech face.
You are correct that the breech face is wider on the 21, but it still locks up just as tight with the G20 barrel in it.
The lock up is not "as tight." With the gun assembled and in battery, I can move the barrel with my finger due to the increased clearance. I can't do that on a 21 or 20 with the respective proper barrels.
You can't use the 20 barrels because the 21 has a larger opening in the front of the slide,.
Actually the barrels have identical O.D. They are the same.