That's not true. If you look at http://kangax.github.io/compat-table/es6/ you'll see IE11 preview (their "canary") has more green than any other browser including Chrome or Firefox. Yes, even more than Firefox nightly or Chrome canary.
while that's sort of true, the big distinction is that the features in their "canary" aren't going to hit "stable" for another year or so, which is a lot of iterations of Firefox and Chrome in the meantime.
However, it is fantastic to see where they're headed with IE12.