Are Emerging Markets Diversified?

GMO's most recent yearly forecasts seem like what most people think - negative on most bonds and most US equities, but most positive on emerging market equities (but interestingly, not their debt):

The traditional rationale for this is that emerging markets (Brazil, Russian India, China) are growing faster and will take a larger slice of the world economic pie going forward. More growth = more profits and more results for investors, right? It seems so obvious and straightforward, but how true is it?

First, emerging markets did outperform up to 2009:

However, looking at the composition of such indices, we see that up to 50%+ is essentially China (Taiwan, even Brazil because of commodity dependence):

As such, is emerging market investing essentially investing in China? Given the the relative short period (1-2 decades vs 1-2 centuries for the USA), are we assuming too much about emerging markets diversification?