You're already aware that the flight crew followed ANA's security protocol.
Yes the flight was closer to PANC, for example. Approximate location based on average ground speed and time it took to return to KLAX is shown below (to make sure there weren't any map projection antics).
Security protocols are not public, but we can deduce from the timeline of the events what had happened.
The cabin crew became aware first (not the airline ops), which means the passenger was likely to be the one to bring the issue to them. Boarding a different painted airplane nowadays than the one on the ticket is no biggie with code-sharing and whatnot, honest mistake.
"While" on the way back, the airline ops discovered it was a mistake, when exactly, no reporter says. Maybe it was soon after.
Let's flip the question.
Why not KLAX?
- Does this passenger have their [perhaps suspicious] checked-in luggage on board? Probably not.
- Can we search them and their carry-ons? Yes.
- Did they come forward? Possibly.
- Do we have handcuffs on board? Yes.
- Can we overpower this one person? Yes.
- Are we willing to arrange (logistics) for their accommodation and ticket back to KLAX? No.
- Do we have different flight and cabin crews to takeover the plane in PANC if needed (duty hours)? Hard to arrange.
- How about arranging for the whole plane accommodation until said crew arrives? Harder to arrange.
Then proceed with protocol. Reacting and overreacting are different. In conclusion, there was no rush to quickly get rid of the passenger.
Addendum: Here by coincidence Alaska is also part of the US, a different route means you're going to burden another country with unneeded trouble, and as the police confirmed, there was no security risk (while the plane was headed back).