I would say her body structure may look slightly similar to a Thoroughbred, or one off the track, seeing as how tall and lanky she is.
But looking at her head and neck, she is surely a TWH. She definitely has the prominent head and neck of one.
I could see how they might think she would be a cross, but honestly, a good eye I'd think would be able to pull her TWH characteristics out enough to see that she truely is a TWH.
Because yes, in ways she could resemble a OTTB, but there are a lot more ways in which she is similar to her real breed...
I don't know. That's my two cents. :)