I think the most important point here is that you should hire engineers from your domain, but not necessarily your exact technology stack -- in their case it's simple because almost no engineers know Erlang well, so they have to hire "database people" and convert them, but I think it's more generally applicable.
Personally, I would much prefer to hire a "web person" regardless of the stack they're used to than, say, a "Java person" who has never written a website. But even more specifically, I would look to find somebody whose interests are in my domain, as they will always have better ideas and motivation than a strictly mercenary engineer.
Not to mention the fact that most experts will spend their time on concept on design rather than code cutting. I think Rob Pike said it well when he said he didn't event have commit privileges at Google while working on Go because he still coded in K&R C.
Personally, I would much prefer to hire a "web person" regardless of the stack they're used to than, say, a "Java person" who has never written a website. But even more specifically, I would look to find somebody whose interests are in my domain, as they will always have better ideas and motivation than a strictly mercenary engineer.