Now, I'm fairly certain that he is, in fact, Garth of Izar as a couple of others believe. Garth was a member of Starfleet...
So were a lot of other people.
Is John Harrison a contemporary of Pike?...a contemporary of Pike...
Is John Harrison a masterful tactician?...a masterful tactician...
Is John Harrison any of that? (He may be a terrorist, but we don't know what his motives are or if he's mentally unbalanced.)...and a looneybird bent on genocide and universal domination who was eventually locked away.
What? Why would he have an assumed name and be walking around looking like Benedict Cumberbatch when he really looks like Steve Inhat? Why would the casual moviegoing audience understand or even care about something like that?The fact that he learned how to change his shape fits in really well with having an assumed name (which only makes sense since there wouldn't have been any reason to keep it a secret for so long if he was in fact a new character)...
Is John Harrison an explorer?...that he was an explorer...
Garth of Izar is not a recognizable character from TOS, outside of hard-core TOS fans. A recognizable character from TOS would be Kirk, Spock, McCoy, Scotty, Sulu, Uhura, or Chekov....that he is a recognizable character from TOS...
Again, why would John Harrison need a pseudonym, if there's no real payoff for the casual audience to get the Garth reference? Did any Star Trek bad guy, past or present, need a pseudonym?and that revealing one of his pseudonyms is a way to give people what they want without letting the actual secret out all fits.
Benedict Cumberbatch and Steve Inhat look about as much alike as Anton Yelchin and 60's Walter Koenig.The fact that Cumberpatch bears a (very, very slight) resemblance to him is just icing on the cake.