You don't become The Oracle of the Transfer Market without having a wide network on spies and informants on the ground, who supply you with information such as this.
He couldn't possibly have any other business there, right?
The full report qualifies the tweet with "if there are no dramatic developments", which has basically been the case for the past month or so, if not longer. Conte to Chelsea seems to be one of the worst kept secrets in football, so were it not to happen, it would indeed be fairly dramatic.
In any case, signature expected tomorrow then. No word on any official announcement, but presumably it will happen at some point between now and the end of the season / start of the Euros. Hopefully sooner rather than later, so we can get on with the whole evaluating and reshaping of the squad business.