I do not think you read the post correctly, "it's interchangeable with the .30 Mauser of "Broomhandle" fame" means that it can be interchanged with the .30 Mauser, that means that it can be done both ways, interchangable: permitting mutual substitution. No question, that is a bone head remark for a paid (read professional) guns writer.I don't see a problem in what he said. He is saying that the PPSH can fire 7.62x25 and .30 Mauser. Which is true and safe.
its no differnent then someone saying a .327 Magnum gun can fire .32 S&W Longs. Doesnt mean it works the other way.