Thanks, everyone. Wanted to update this post in case anyone ever comes along and asks the same question. After a bunch of back and forth with Wix, it looks like there's no solution. Their response: "Unfortunately we are unable to provide support in regards to HTML iFrames and responsiveness on mobile."
They suggested trying to drop the iframe into their "strip" content block which supposedly *is* responsive, but that didn't work for me either. Maybe if I knew more about CSS and could hack this myself there would be a way around it, but out of the box, there isn't a good way to host Twine files on Wix.