Read his post again: "There are strains of an almost Lovecraftian incomprehensibility to many gods and demons".
You're taking a statement that essentially says "many D&D gods and demons have an incomprehensibility to them, almost in a Lovecraftian way" (note the use of "strains of" and "almost" in the post), and reading it as "Mike Mearls thinks Old Ones should be everywhere in D&D."
What I am saying is why can't they?
Deities and Demigods may have used two carefully written and copyright works, but it doesn't mean that they were not inspired from real world mythology, that can be placed into D&D.
I think it would add a bit more flavor to the gods than what exists, and that is something 4th edition sorely lacks is some flavor. Something to inspire you. Something "old school" gaming did. Be it going to look up these things for yourself to read about, or to create your own original ideas.
I was interested in what little viking information I had access to, but getting my hands on D&D made me want to look more into it, as well as Aztec and other mythology.
I think that added inspirational material would go a long way to getting back some of the old school feel and style of play. That kind where you want to know more about the world you are in, rather than trying to create the latest fantasy movie within your game, but create your own story with bits and pieces from many different things.
Lovecraft was a good one for taking old ideas and giving them new life, so what is wrong with D&D doing it? D&D cannot contain pagan or druidic old work references anymore from a mythological aspect, but can still have breasted dragons cutting the heads off of humans?
Has D&D become Lion Witch and Wardrobe fantasy like Charlie Brown with strong Christian overtones, and cannot deviate from that idea into real world mythology, but must create its own fake mythology?