Right - but when one is conducting casual research, one plays the odds, and the odds are heavily on favor of the Britannica and the OED, etc. over Wiki-anything, and blogs. And the thing about poor old King John is a bit of a cavil; true he didn't write his name on it, but he did indeed "sign" it in the sense of "validated it by affixing his mark." I suspect one could find more than one source that would support that sense of the word "sign."