Didn't Harry, in fact, do this in an earlier book, which is how we first got acquainted with the sword? And I'm fairly sure this was mentioned in HP7 in re Neville, briefly, once.

...I agree it was confusing. I definitely had a moment of "how did he get that from the goblins?!?" I think Rowling lays the groundwork for everything she does, but sometimes expects a little too much of our detail memory, after seven books and a decade.