I don't agree entirely. A slice box can be an outlet, but in the case of extending a circuit like that, it is just a splice box. The outlet is now at the new end of the extension. Now connect a load at the splice box and you make it an outlet again.
The definition is not specific enough though to say either of us is wrong though, but every box that contains splices could be called an outlet box, and I don't believe that is the intention, I think the main intention is the final transition point from premises wiring to utilization equipment. Some "outlets" are not even in a box but rather are either the last point of attachment to the premises wiring or the line side terminals of the equipment supplied, which one is correct isn't really defined.