It seems to me (from a brief scan of the spec) that they actually are using 802.15.4 to establish the mesh at low-rate speeds, then switching to a different high-rate standard once the handshaking is complete.

Given the rate limitations of Thread (I'm assuming they're similar to the limitations of the underlying 802.15.4 standard), then it really isn't solving the same problem as this new specification, but are actually complementary.

Perhaps GP meant that existing high-rate mesh implementations are vendor-specific?