I only skimmed that blog post after a while because it reads like a pitch from an MBA targeted at venture capitalists.

I feel like thats not actually a distinction that matters to application developers, because they know thats just a technical detail that only concerns the developers of the database. Ultimately, all the compatibility has to be implemented in the storage engine. The fact that they're using Postgres's porcelain is surely a time saver for them, but of no consequence to the consumers/users of the database