Yeah, I've used both MySQL and Postgres in the same app before (legacy transition/migration). There is no issue with Postgres in PHP, and it works just fine with, e.g., PDO.
This was a real-time interview, and I was the one who edited for grammar because Evan had already left. I'm assuming this didn't come out the way he intended it to. I can't imagine (though I could be wrong I suppose) he is under the impression Postgres doesn't work with PHP.
When he gets back I'll have him chime in!
This limitation (of Heroku's?) is fairly interesting to me. I'm curious as to why (perhaps there just isn't enough demand for Postgres/PHP apps in their experience so they haven't developed support for it?).