The leak came from SDNY to Buzzfeed. SDNY has all of the evidence since they’re the ones who performed the search warrant on Cohen.
Your point is valid and should be extended:
A) SDNY leaks like a sieve. It always has leaked like a sieve.
B) The number of people including Investigators and Prosecutors actually read into a case is always greater than your average citizen would expect it to be. That is not to say that those read in should not be read in. It is just to say that the average joe would likely be very surprised how many investigators end up read into a big case.
I think Mueller reacted for two reasons:
1) He wanted to immediately address this nonsense from the Hill suggesting that "Mueller has to finish and report to us now". I will repeat something from a different thread. Mueller is a Special Counsel not an Independent Counsel. His entire means to exist is Indictments and Prosecutions. He does not get to report directly to the Congress as an Independent Counsel would. His report is to the AG and even then if Mueller sticks to the design of said report it would not by nature yield the kinds of things that Congress is looking for. Even at THAT, then the AG gets to decide if he will report anything publicly and he will decide what goes into whatever he might report publicly. So first and foremost, IMO Mueller wanted to shut down this "Mueller must finish now" nonsense from the Hill now coming from some Dems as stupidly as it has come from some Republicans.
2) IMO, Mueller wanted to tamp down any nonsense about Leaks coming from his Investigation and he wanted to do it for his own Investigators that might start looking oddly at each other if they thought one of them or two of them might be leaking and he wanted to establish that for the rest of us as well.
I am disheartened that the two Buzzfeed reporters on the byline have contradicted each other in their initial comments about corroboration. Buzzfeed's boss and one of the reporters on the byline were on this morning and they only sort of answered the question about that contradiction. At least THEY should be treating such a story with the seriousness it deserves and they should have been prepared for the simple fact that the corroboration questions would be the important questions they would face immediately.
They claim this morning to be confident in the story and they claim their story will be born out in time. They are really hung out in space on this one. Again, Mueller's only real function is to Indict and Prosecute. If he is not going to Indict and Prosecute the President while he sits as President it may likely be a long time before if ever that corroboration comes from Mueller and I am inclined to think SDNY is pretty much done as regards this specific aspect of the various cases floating around out there. It might come out during the Indictment and Prosecution process for others that have not been Indicted as yet as they might give up Trump in the same way that Cohen appears to have.
Might it come through Congressional Hearings? Yes, but not likely while Mueller's Indictments are still in process though it might come out after all the Indictments are in and the Prosecutions are in process.
Buzzfeed said this morning that they would not expose their sources but that they had gone back to them and Buzzfeed is reporting that the story is still solid.
Finally, comments that the Mueller statement amounts to a definitive rejection of the Buzzfeed story are sheer nonsense. Go get yourself an English dictionary and a few lessons in English grammar if that is a case anybody wants to make.