That poll is trash, Cruz will win Texas vs O'Rourke, it will probably be something along the lines of 55-42. I think that O'Rourke will slightly over perform the average democrat in the state, and Cruz will substantially underperform the average republican in the state. Just because of how sucky Cruz is, and O'Rourke is pretty good, I think those margins will be a little thinner than usual, but at the end of the day, it is Texas.
I agree Cruz would beat O'Rourke. But, I think Castro has a real shot of pulling the big upset, given its an off year, Cruz not very popular, Trump's low approval, and Castro's potential to motivate latinos, millennials, & women's march voters in a low turnout election (28.5% turnout in last off-year Texas election).