If you have a list of search results, and not all of them are on screen, it is necessary to scroll. I don't see what Vimium has to do with it, other than using j and k instead of the arrow keys.
In any case, certainly you can recognize that a huge proportion of people scroll with a mouse, n'est-ce pas?
Sure, but on the other hand I recognize that DDG might not want to signal that its results are so useless that you need a prominent Google fallback right on the page. For me, it's not even the case, I rarely search with Google and I've been using DDG for years.
I suppose I get that. However, allowing the established player as an easy fallback when you're the underdog usually helps adoption rather than hurting it.
Look at Mac sales after Intel/BootCamp/Parallels. VirtualPC existed before but it wasn't easy or fast. When it became almost seamless to fall back to Windows when necessary, more people were willing to take the leap.
In any case, certainly you can recognize that a huge proportion of people scroll with a mouse, n'est-ce pas?