Malloch revealed later that the FBI questioned him extensively about Roger Stone, Trump's close friend and long-time advisor, and possible visits to the Ecuadorian Embassy in London to meet Wikileaks leader Julian Assange, widely known to be a conduit for Kremlin leaks of stolen emails.
For its part, yesterday NBC did further reporting on The Guardian scoop and fleshed out more details (link here).
For quite some time, a number of prominent Washington insiders have openly discussed the possibility that Stone served as a "cutout" in a chain of back-channel contacts between Trump's campaign and Vladimir Putin and other Kremlin officials.
This news thus suggests that Mueller is attempting to make the case that the Trump campaign colluded and coordinated with Russia on various hacks of tens of thousands of emails from both the Democratic National Committee and also Hillary Clinton's campaign.
If such a connection is proven, that would establish that a number of felonies were committed by individuals in the Trump campaign and possibly Trump himself.
At this point, this news only provides a tantalizing clue of something potentially explosive, the proverbial smoking gun. Hopefully more will be leaked about this development soon.