In this case there's little lock-in, since it maintains InnoDB interface compatibility - you could drop-in replace it with, say, a DigitalOcean-hosted MySQL instance, if you wanted. (Of course you'd be missing out on other perks, but your core MySQL integration wouldn't suffer lock-in.)

Yes, not using any vendor's products protects you from vendor lock-in. To call that "generally preferable" is awfully simplistic though.