Read the proposal and critique carefully. Note the part about how the app is useless if you're the only person you know who owns a copy. Then note the part about the Android platform, the one platform on Earth that (for the moment) is guaranteed not to be owned by a single one of your friends. Finally, note that PG's critique is quite carefully phrased.

It's frustrating enough to develop an app that depends on the adoption rate of a vaporware platform, but it really is kind of crazy to develop an app that depends on the square of the adoption rate of a vaporware platform.

Still doesn't make it the worst idea ever. Hell, you could have an app which depends on the cube of the adoption rate of a vaporware platform.