I don't know where you got that definition, but it's not the way the term is used here.
The term 'purist' was originally spat at we Freepers who refused to follow the liberal pied piper known as Mitt Romney, in 2012.
We were actually accused of treason to the country, because we chose to stand by our conservative convictions when most others were being terrified into throwing theirs out the window to support the 'lesser evil'.
I concluded a few years ago that many of those on FR (certainly not a majority) who espouse “purist” views do so in order to encourage others to not vote at all in elections if the candidate does not fit their idea of perfection. Many here stayed home and didn’t vote for McCain or Romney - one reason we have Obama and the resulting unmitigated disaster. Obviously many of these people are principle conservatives who conscientiously can’t vote for a McCain or Romney, but I think many of them are democratic trolls.
We are not looking for purity, just a modicum of actual conservatism. Someone who will actually move the ball for the conservatives.
I would add that in my opinion if Trump is even willing to allow the GOP to believe that he is potentially willing to go third party, then he is unsuitable. Because even allowing anyone to believe that he is willing to do so is tantamount to suggesting that he is willing to increase the probability that the democrats win the election. Make no mistake, if Trump goes third party, the democrats chances of winning go up. There is no excuse for that.