I'm thinking this is a vote trade between Cruz and Cornyn. As a member of pubbie leadership no way can Cornyn vote against the open borders the donor base paid for. Conversely, Cornyn can't get re-elected in 2020 if he doesn't vote to uphold President Trump's emergency order to close the border.
Cruz just got reelected last year so he's safe for five years and needs to accumulate some brownie points with leadership. The open borders crowd won't hold Cornyn's vote against him as long as the border wall gets scuttled. Pretty slick, huh.
I'm sure you're right. Good thing Trump held that rally for Cruz to drag him across the finish line. At this point I almost wish I'd voted for Beto.