It does scale when being considered by software running on it. Even more with some alternative approaches like directories. What OP means is that it does not scale for software to be oblivious to underlying cache coherence and to operate on "one flat shared RAM" assumption. E.g. false cache line sharing etc.
This is a controversial statement, and an opinion that is not shared by many respected computer science researchers: http://research.cs.wisc.edu/multifacet/papers/tr2011-1_coher...