Who was Hans-Dietrich Genscher? He was Germany's foreign minister, who had no authority to promise anything on NATO.
Once again, you do not address the fact that Gorbachev has said in subsequent interviews, that there was no promise not to enlarge NATO eastward past Germany. Also, former Soviet Foreign Minister, Eduard Shevardnadze also agreed, said there was no limit on NATO enlargement.
What time frame are they talking about in that memo? If it was before the reunification of Germany, before the dissolution of the Warsaw Pact and the end of the USSR, it’s one thing.
After the nations of Eastern Europe ceased to be vassal states of The Soviet Union, they were free to enter into whatever treaties they wished.