Linguistic subterfuge. Every time they tie the proceedings to "unprecedented" language, they use the term "Investigation". Every time they refer to the actual thing that has begun - the Inquiry - they use the proper term, "inquiry". And inquiry is not an investigation, and is not bound by the same principles. It's the thing you do before a trial, to make sure you have all the pieces and parts to the whole puzzle.
The problem is that there won't be anything resembling a trial in the Senate. Mitch is gonna say something like "And here, now, we will discuss the articles of impeachment: They have been sent to us by the House on a more-or-less party-line vote. Now that we have discussed these cLeArLy PaRtiSaN impeachment articles, we will take a vote this instant, because we need to acquit this poor, persecuted dictraitor so he can get back to fucking shit up, stat."