I'm guessing there was a security imperative to return immediately.
If his checked luggage inadvertently went on a different flight perhaps that set
off alarm bells.
If checked luggage on a different flight was the main concern, wouldn't the first imperative be to turn that OTHER plane around - the one carrying his checked luggage yet with no corresponding passenger aboard?
Perhaps had no checked luggage. Or was removed from the other flight when he failed to board it. Either way... the checked luggage issue doesn't seem grounds for turning ANA Flight 175 around.
But, as you say, "Security procedures aren't known for their flexibility."