The reason that the guy gave in the extensive article he wrote was that the same load of shot in a twelve gauge was a rounder, bulbous and short column, and sure, if that pattern struck dead center it will smoke the clay. The twenty has a longer column. If it has the same pattern diameter on paper,in reality, the long column functions to m make the pattern into a longer oval, when the time of passing through that longer column is extended.
Two things to consider. This was written when shells had paper hulls, cork wads, and a paper cap sealing the opening. Maybe even a roll crimp with cork overshot wad.
Second thing to remember is that the guy earned his bread by writing. If he wrote something interesting, even if it was stupid, he earned a week of food and a mortgage payment. I've read articles that said that a .32 was a fantastic carry piece, I've read that the .270 was better than the .458.
If people didn't write some controversial nonsense that gets attention and readership, magazine sales flag, sponsorship drops,ad revenues decline, people don't care. A lie always manages to benefit someone, or there would no reason to lie.