In the comics, Roy Stewart was the grandfather of Green Lantern John Stewart.
Presumably John Diggle kept his father's surname when his mother remarried. Had he taken his step-father's name then he would be John Stewart, matching the name of Green Lantern. In Elseworlds, Part 2 (2018), it was established that on Earth-90, John is Green Lantern. It may be that on that earth, he did take his step-father's surname.
Alena mentions a Doc Magnus, in the comics Doc Magnus is the scientist who created the Metal Men.
The weapon the Ninth Circle goes after is reminiscent of the missiles in Michael Bay's 1996 film "The Rock".
Stewart's Marine Corps serial number is 498003145.