IIRC negotiating is an executive function - ratification is the job of the senate ...
Further - most treaties are "secret" during negotiations -
Except the copyright cartel has had a front-row seat throughout the process. The treaty basically reflects their desires, and nobody else's, as they're the only ones who had input. Later, about 40 people were allowed to see what was going on, and they were all from copyright-based corporations with the exception of a few token people from public interest groups.
As far as bypassing Congress, that's the intent. They don't want the trouble of flat-out passing such draconian copyright laws that are so clearly pandering to special interests. But if it's a treaty they can present it as a fait accompli, then it will be easier to enforce the copyright cartel's will.
That's how they did the last copyright extension, the CTEA, a.k.a., the Mickey Mouse Protection Act.