You make a good point that the fact that there are different results doesn't mean that they are personalized. It could simply be that Google is providing different search results so they can get better data on what people want to click. This would be a benign — indeed legitimate — explanation for the DDG study results.

However, there isn't evidence that this was what was happening, and Google hasn't exactly covered itself in glory on the privacy front so I tend not to give them the benefit of the doubt on this front. I'd love to see a response from them addressing some of the points from the DDG study.