You're right I have no idea what DDR means in the context of countries or geopolitics.

As for the claim that NATO wouldn't expand past the DDR, all I could find were sources that say it was an oral agreement between the US and the USSR. I don't see why the USSR (now Russia) should get to decide what unions eastern European countries decide to join, especially since after the collapse of the Warsaw pact said countries were no longer part of a defensive pact.

https://www.politifact.com/factchecks/2022/feb/28/candace-owens/fact-checking-claims-nato-us-broke-agreement-again/

https://www.npr.org/2022/02/07/1078929982/a-look-at-the-debate-over-nato-expansion-eastward-thats-at-the-heart-of-conflict

https://www.france24.com/en/russia/20220130-did-nato-betray-russia-by-expanding-to-the-east