If Clinton is winning 28% of Republicans, she will win 40-45 states (including Texas). It means either this poll is wrong or every other poll is.
I think you're misunderstanding the results.
TargetSmart isn't claiming that Clinton is going to win 28% of ALL registered Republicans, just that she's won 28% of the roughly 43% (give or take) of Republicans who have already voted early.
Presumably that's because a lot of the Republicans who are actually angry enough to vote against Trump (as opposed to not voting at all) were motivated to do so early.
But if Hillary only gets, say, 7% of the remaining 57% of Republican votes, than she'll wind up with roughly 16% of the overall registered Republican vote.
Which is still a lot, but not necessarily unbelievable due to the large number of Cuban-American Republicans.