Speaking as a Travis County resident, this is not a difficult question. If your survey of Hispanics in Texas is limited to Hildago, it is by definition unrepresentative of Texas as a whole. If your question is, "why are Latinos in Texas going Trump", then it behooves you to go into places like Travis.
Hildago tells you very little about the swing. I don't go to Wyoming to explain the 2008 Obama election. Same principle here.
Now, if I were doing a poll with serious systematic issues to reassure people that yes, Hispanics are still democrats, I would do exactly what they have done here.
