Per the article, it's probable that the styled emails are being flagged as promotional (or outright spam) and thus being filtered from immediate view by various mail clients (including Gmail).

Yes, it seems the open rate should be attributed to the subject line or the a/b split parameters more than the design...

And with the open rate that significantly affected, all other metrics would be skewed as well.

Not necessarily the full story. I have set my preference to text email over html email in one client that I use. This is at least one example of a client making the decision before opening the email.