I believe that is really was Jesus showing Himself to John.
The Visitor goes on to say to John, “I am He that lives, and WAS DEAD…” (Rev 1:17). No angel would have been able to say that. I think we can also say with strong confidence that it was not Moses. Because while Moses “WAS dead”, and showed himself to be ALIVE again (at Jesus’ transfiguration), Moses does not possess the “keys of hell and of death” (Rev 1:18.) Only God can release people from “hell” (the grave), and from “death” itself. Only God has the “keys” of hell and of death.