No single benchmark for the web
hacks.mozilla.org
hacks.mozilla.org
Let's not forget the good Mozilla has done in making a free and open web. Also, Fantastic Mr Firefox recently becoming a worthy competitor on technical merit alone, is a truly inspiring milestone.
As OP points out, in order to optimize, there must be something to measure. This is Google saying the benchmark is what they'll be using as a measuring stick internally (and likely, in their marketing against competing browsers).
Does all of this make the benchmark a useful tool for Google? Yes.
Is it the best benchmark for cross-browser comparison? Probably not.
Second, while Emscripten is a Mozilla project, you can see that it works with Chrome as well: Links to bugs filed by an Emscripten dev (me) appear in that blogpost, in fact - those bugs are filed to help Chrome be faster, and nice progress has been accomplished, one of those bugs was just closed in fact (it's cool stuff, read it for more info - Chrome was much slower than Firefox on that benchmark, and now it's faster on it).
Finally, the point of the blogpost is that all benchmarks need to be treated skeptically - there is no such thing as a perfectly unbiased benchmark. So we need to just be aware of that, and not consider any specific benchmark to be "the" benchmark of the modern web. That's all.