I guessed wrong too, which was a surprise to me when I looked this up:
“New filming took place for the framing story for “The Cage”. Since actor Jeffrey Hunter was unavailable to reprise his role as Captain Pike, a look-alike actor, Sean Kenney, played the injured captain in the new scenes, although Hunter was represented in flashback footage and credited accordingly (along with the other original “Cage” cast).” source
I guessed wrong too, which was a surprise to me when I looked this up:
“New filming took place for the framing story for “The Cage”. Since actor Jeffrey Hunter was unavailable to reprise his role as Captain Pike, a look-alike actor, Sean Kenney, played the injured captain in the new scenes, although Hunter was represented in flashback footage and credited accordingly (along with the other original “Cage” cast).” source