WARNING: Check the loading manual.

[Mr. 16 gauge]

At the risk of showing my ignorance, why would he use a "shotgun battery cup primer" in a rifle metallic cartridge case?

Most likely because the pressure caused the case to expand and deform that much.

I have seen some HORRENDOUS overloads listed on various sites on the internet; actually pulled a few off when I was a moderator because they were dangerous, then got lambasted for it from the poster...."I've used that load for years, You don't know ****, ect., ect." If they indeed did use that load for years, then they have a very active guardian angel!
One needs to remember that when dealing with loads off of the internet, it is very easy to hit the wrong key and hit a "2" instead of say, a "1".......and most folks don't proof read their posts for typos!