I'm sorry, I inaccurately paraphrased - a more accurate statement would be that he appears to perform better in open primaries.

You're right that Overwhelmingly well was a massive oversimplification and exaggeration.

More accurate would be "appears to perform worse in closed primaries"

http://www.vox.com/2016/4/28/11528764/closed-primaries-berni...

https://www.bostonglobe.com/news/politics/2016/04/19/bernie-...

I can't see anything in your link that clearly indicates your position WRT clinton overwhelmingly winning open ones.

Wiki does not list open primaries, her win proportion of those, or the 'goal difference' in terms of awarded delegates. So - I'm not sure your statement is entirely true either.