In May 1990 speech Secretary General Manfred Wörner said “The very fact that we are ready not to deploy NATO troops beyond the territory of the Federal Republic gives the Soviet Union firm security guarantees.”

This shows 2 things:

1. As we already knew NATO had told the Soviet Union although that it would not move further East than East Germany
2. They understood full well that NATO moving East was seen as threatening by USSR given the promise not to do so was a “security guarantee” for them.