That's just a standard socket mate without the insert. Not saying it doesn't fit. Just saying it's NOT a spark plug socket. Spark plug sockets are special purpose sockets used only for spark plugs.
I know that's what's always confused me about them. I think it is the o ring diameter too. The nut part probably is 16mm. I've never used a standard socket for plugs though lol.
There's a picture for you mate. See the size differences for the same size mm plug. The big 10mm (with a spark plug in it) is a proper spark plug socket not a normal socket.