Yes, you are free to go out of the terminal at NRT and wonder around the city. When an airline says “though check-in” that means at ITM your checked baggage will be tagged all the way to LAX and do not see your checked baggage at NRT. Also, you will receive boarding pass for your NRT-LAX flight at ITM.

Your situation is ITM-NRT on domestic flight then NRT-LAX on international flight. I am assuming tha both ITM-NRT and NRT-LAX are on NH (ANA). Upon arrival at NRT as you walk way to arrival hall of the terminal, there will be “International Transit” or "International Connecting Flights" area. DO NOT GO THERE. Yes, that is for passengers transferring to international flights. However, that is departure passport control for international flights, and once you go through that you will come out on international departure gate area and you will not be allowed to go back out of the gate area.

Pass by “International Transit” or "International Connecting Flights" area and continue as NRT is your final destination. You will come out on arrival area of the terminal and you will be free to go to the city.

Once you come back to the airport, head to departure level (4th floor) of Terminal 1 South and you will go through standard airport security and international departure passport control, which will lead to departure international gate area.