That’s what I’ve been assuming. Either someone in the room with him or he recorded it.
Or..because it is DC..and people cover their butts and back stab...someone recorded the phone call and worked with Mitch to out it right when Mitch is trying to strong arm and bribe Senators.
Or Trump recorded it and leaked it himself...he did infamously do something similar in the past before he became POTUS...even pretended to be someone else when talking about it.