It says it wasn't a "formal alliance". The Nazi-Soviet pact was alliance for practical purposes as seen with joint "victory parades" with the Wehrmacht and Red Army and expanded trading.

I'm not sure what specific "rebutted by historians" you're talking about. There were disclosures about Nazi-Soviet secret protocols in 1946 : https://en.wikipedia.org/wiki/Molotov%E2%80%93Ribbentrop_Pac... though Soviet attempts to deny this were not credible: https://en.wikipedia.org/wiki/Molotov%E2%80%93Ribbentrop_Pac...