No, they didn't. 52% of those who voted on Prop 8 (about 79% of the ~17M eligible and registered to vote, which were themselves only about 45% of Californians) voted for Prop 8 -- so about 19% of Californians voted for it.
> so you could argue Brendan Eich holds a rather popular opinion.
Well, no, you couldn't make a good argument that he holds a rather popular opinion based on the results of an election 6 years ago. You could perhaps argue that he holds a view that was popular at the time of the election based on those results. After all, opinions change. [1]
[1] see, e.g., http://www.ppic.org/main/publication_show.asp?i=1012