This often indicates that practice deviates from policy. The policy is that you should have a T5 departure boarding pass to use those lounges. In practice you may be admitted without one, either because attendants don't read carefully or it isn't important. Just be prepared for the possibility that it won't work out.
Good news! It worked out for us.
We had to take a shuttle bus from Terminal 1 to Terminal 5, and then go through security again. This all took awhile, but with a 6 hour layover before a 13 hour flight, stretching our legs and getting some good food was well worth it. Anyway, they asked for my boarding pass and looked at it closely. Maybe they only checked the Date, or maybe they don't care about the Gate, but we made it in. Thanks,
--Kyle