He doesn't "diss UFAI concerns", he says "I don’t know how to productively work on that", which seems accurate for the conventional meaning of "productive" even after looking into the issue. He doesn't address the question of whether it's worthwhile to work on the issue unproductively, illustrating his point with the analogy (overpopulation on Mars) where it's clearly not.
His article commentary on G+ seems to get more into the "dissing" territory:
Enough thoughtful AI researchers (including Yoshua Bengio, Yann LeCun) have criticized the hype about evil killer robots or "superintelligence," that I hope we can finally lay that argument to rest. This article summarizes why I don't currently spend my time working on preventing AI from turning evil.
http://fusion.net/story/54583/the-case-against-killer-robots-from-a-guy-actually-building-ai/
(An earlier version of this article was titled 'Andrew Ng disses UFAI concerns" which is the phrasing many of the commenters are responding to).