It is Gorbechev that knows more about the discussions than Flynn.
1) Your source on this is left-wing Brookings Institute.
2) It's source for Gorbachev's alleged comments is something called Russia behind the Headlines. But, there is no active link to the interview. So, we don't even know if the interview actually took place, let alone if it was translated unbiasedly from Russian to English.
3) Even the Brookings Institute concedes this: "To be sure, the former Soviet president criticized NATO enlargement and called it a violation of the spirit of the assurances given Moscow in 1990."
So, we can believe this dubious source or declassified intelligence documents that say we promised Russia no eastward expansion of NATO.
The bottom line is that you believe we and NATO had the right to aggressively and ruthlessly threaten the Russians without consequence all so politicians could make $$$, George Soros could make $$$ by setting up NGOs in countries like Ukraine and so Russian hatred could keep money flowing to the defense industry.