I don't think this information aids at all in making that determination. He could have been suspicious of any of the major candidates. Besides, it's not exactly "new" information because he'd said before his initial reaction was "huh, those people were right" rather than "we were right". By distinguishing the correct guessers of Asmo's killer as "those people", you can automatically infer his own primary suspect (if he even had one or the inclination to care overly much) was not the killer.

And then of course, there's the fact that he's shown a very poor grasp of the details of the series without consulting notes or Maria, occasionally even confessing he accidentally gives wrong information to fan questions, that it's probable his deductions would be in line with those who didn't pick the most popular/obvious culprit. Honestly, the man isn't perfect, he doesn't have a photographic memory and, prior to his gaining access to RJ's notes, we shouldn't expect him to get all the minutiae of the series correct any more than your average fan would.

Even if Sanderson had declared the opposite and said "it's nice to know I was right", I wouldn't claim that it implicated Graendal more than the other candidates. His being wrong or right or even disinterested helps in no way determine who the killer might or might not be. Hell, I know perfectly intelligent, reasoning people whose opinions I normally respect that think Lanfear killed Asmodean, which is a candidate I consider extraordinarily unlikely. For all I know, Sanderson was in their camp.