His sentence does seem misleading to me: it reads like he thinks Windows at the core OS level (the scheduling/memory-management/drivers/etc. stuff he mentions in the intro) is still incrementally built on top of the old DOS-lineage code, as opposed to recognizing that it underwent a complete rewrite. If he's just arguing that the API evolution has been constrained by the need for backwards compatibility, that's true, but doesn't make much sense to his point in this context.