Noble’s casting is a bit strange, but still explainable, given Cisco Ramon’s shirts and other pop-culture references on The Flash indicate shows like The Big Bang Theory and others exist in the Arrow-verse.
It’s really a shame John Noble isn’t appearing on The Flash, since the Arrow-verse is missing a golden opportunity to team-up Noble’s Denethor II with Wells The Gray!
News of a real-world actor playing himself in an Arrow-verse show may be surprising, but it should be expected.
Legends of Tomorrow has featured existing past and present figures like George Lucas and J.R.R. Tolkien in its adventures, although those instances featured actors’ portrayals and not the people playing themselves.