This is incorrect usage of the word "corner" to be overly pedantic. It's still a corner, but English doesn't have a separate word for inside vs. outside corner,

This is incorrect usage of the word "corner" to be overly pedantic. It's still a corner, but English doesn't have a separate word for inside vs. outside corner, so it's common to just say "outside corner". And yes it is a corner (or vertex), where two planes meet. Edge can also be correct, but corner is not wrong.