I can't comment on that particular test, but the problem with concepts like Mary Sue and Purple prose isn't that they contain poor element, but that they are good elements used poorly.
What makes purple prose bad is not that it is a grant writing style, but rather it is a grand writing style handled poorly. Words are fickle, rebellious things and it takes a skilled trainer to get them to perform flawlessly.
It is a similar situation for the Mary Sue character; no writer can avoid placing themselves inside the story. When it is done well, it is a pleasure to read. But when it is done poorly, then the reader wishes to follow Oedipus' example. Mary Sue isn’t when the characters are larger than life, it is when the larger-than-lifeness of the characters are not earned through the storytelling. Good characterization might be like a super bowl commercial; totally fantastic and unrealistic, but not particularly annoying. Mary Sue characterization, however, is more like any commercial with this guy in it:
Or any commercial in which individuals fail at basic activities, like using a blanket:
http://www.youtube.com/watch?v=h05ZQ7WHw8YThough to note, it seems like there is some divergence in what I understand a Mary Sue character to be and what you have encountered. As I learned the term, a Mary Sue is a character with no flaws and to who all good things occur. A tragedy with a Mary Sue character at its center would be a strange thing indeed, as almost by definition nothing bad can happen to a Mary Sue character (at least, not during the story; before only increases their mystic so that is all good).
Achilles, for example, should automatically be discounted from consideration as a Mary Sue character since, though he is important to the Greeks and all their hopes rest on him, crap happens to him. That he is doomed to die, that his dear friend dies because of him, that he has few redeeming qualities, etc all go against the Mary Sue theme.
There is, of course, some overlap as modern Mary Sue characters may well specifically attempt to cash in on established clichés (and these clichés were established by the likes of Achilles). The problem there is that clichés aren't actually bad; they're too good. Clichés are so good that they become overused. I like the taste of chocolate, but I do not want it for every meal; in the same way, cliché’s are good, but let us have variety as well.
Like any literary element; when used expertly, the device is good, and when used ineptly, the device is bad. Mary Sue and Purple Prose are just two examples of literary devices being used poorly. Not that the thrust behind them is poor, just the execution.
