[crunchatized]

It's true, the Mueller report is light on actual evidence and in places rather heavy on hedging language.

But it seems not an unreasonable guess in this case that there was possible use of "WikiLeaks's private communication system" based on a Twitter DM that was quoted in the Mueller report:

On September 15, 2016, @dcleaks wrote to @WikiLeaks, “hi there! I'm from DC Leaks. How could we discuss some submission-related issues? Am trying to reach out to you via your secured chat but getting no response. I’ve got something that might interest you. You won't be disappointed, I promise.”