It's also interesting to point out that Cohen's sentencing statement says that he made false statements to congress at the direction of Trump (Client 1).
So while it may be that Mueller is saying Cohen didn't perjure himself before Congress for the reason given by Buzzfeed (ie the particular details about Trump Tower), or at least that the Buzzfeed article contained particular inaccuracies or was likely to mislead readers into thinking that Mueller's legal team was the source, it doesn't mean Cohen didn't perjure himself before congress at the direction of Trump for another reason (eg electoral collusion with Putin). And it may be that Buzzfeed's source was someone other than Mueller's legal team: someone who nevertheless had internal information connected to the investigation (eg an FBI investigator).
In fact, since Cohen's sentencing statement alleges that Trump directed Cohen to lie to Congress the actual material question is this. What is the precise description of the lies Cohen alleges that Trump directed him to tell to Congress?
We know Trump told Cohen to lie about Stormy Daniels. Trump lied about the fact that he had done so. So his denials are meaningless, except in revealing him as obstructing justice.
I wonder what else Trump wanted to be hidden and directed Cohen to hide. And what evidence Mueller has.
As regards the Tower, there is little question that Trump lied about pursuing a deal to build it long after he said he had ceased to do so and while he was a candidate. This is the basis of a prosecution for fairly obvious bribery and collusion (or conspiracy if you prefer).
Whether Cohen lied to Congress about this at the direction of Trump or of his own volition is something that is in question. But whatever the answer to this question is, it makes no difference to the issue of Trump's damaging collusion with Putin for his own benefit.