Likely true, but a third party might be able to peel off working class whites from the democrats if it had a populist orientation, particularly against a phoney like Hillary.
A third party effort would certainly be motivated by disgust at being marginalized by the republican establishment. Basically, a game of chicken. They feel we have no where else to go and will not want to elect Hillary. We feel like if we're going to be ignored, why should we support them? Really, is Hillary that much worse than Jeb Bush or Chris Christie? We survived her husband and Obama, we can survive her.