Here's why Mueller's denial of the Buzzfeed-article could mean anything:

"BuzzFeed's description of specific statements to the Special Counsel's Office and characterization of documents and testimony obtained by this office, regarding Michael Cohen's Congressional testimony are not accurate"


Let's compare this statement with the very first sentence of the article it refers to:
https://www.buzzfeednews.com/article/jasonleopold/trump-russia-cohen-moscow-tower-mueller-investigation



If the lying was Cohen's idea and Trump went along with it, the Buzzfeed-article is not accurate.
If Trump ordered Cohen to lie not to Congress but to law-enforcement, the Buzzfeed-article is not accurate.
If Trump ordered Cohen to lie to Congress not about the negotiations but about something else, the Buzzfeed-article is not accurate.



If the plan was not from Cohen, the Buzzfeed-article is not accurate.
If the plan was not to meet in Russia but elsewhere or in another way, the Buzzfeed-article is not accurate.
If the plan was not to meet Putin but somebody else from the Kremlin, e.g. from his inner circle, the Buzzfeed-article is not accurate.
If the meeting was not supposed to be about Trump Tower Moscow but something else, the Buzzfeed-article is not accurate.
If Trump didn't say "Make it happen", the Buzzfeed-article is not accurate.



And on and on and on. You can do this with every sentence.




And the part where Mueller's spokesman says that "BuzzFeed's ... characterization of documents and testimony obtained by this office, regarding Michael Cohen's Congressional testimony are not accurate"


If there is only one witness, Buzzfeed's characterization of documents and testimony is not accurate.
If the witnesses are not from the Trump Organization but from his campaign or the RNC or whatever, Buzzfeed's characterization of documents and testimony is not accurate.
If the emails are not internal company emails but sent via other email-addresses, Buzzfeed's characterization of documents and testimony is not accurate.
If this is not corroborated by internal company emails AND text-messages AND multiple "other documents", Buzzfeed's characterization of documents and testimony is not accurate.







Mueller didn't say what's wrong or how much is wrong. It could be anything, from the central accusation to a minute detail.