In the past under PMNW, GE and PE with Asian addresses got lounge access. Now on DL, GM and above is now SkyTeam ElitePlus and hence gets lounge access when travelling SkyTeam internationally. This does not depend any more on having and address in Japan.

Many DL offers are targeted and many DL FFs complain that non-USA residents receive few if any of these targeted offers, such as DEQM or special bonus miles for certain routes. One suspects that this iscludes DL-branded credit cards that offer miles; some of these cards are for residents of certain countries. I don't know whether having a DL credit card based on a USA address but a DL FF account with an address in Japan would cause problems. I also don't know whether using a Japanese address would cause delta.com to price flights in yen; this should depend on the originating country, but I've heard that some people are experiencing issues with getting tickets to price in US$.

You should ask your colleague for more details of his DL offers or request that he forward the email announcements of some recent ones to you. (I assume that you can read Japanese.) Could he be thinking of the extensive special duty-free catalogues for delivery in Japan?