According to MKBHD’s video, you just look at the key you want on the on-screen keyboard and then click it by touching your thumb and index finger. They also mentioned in the keynote you can connect a Bluetooth keyboard and type on that.
For the first few minutes I thought Hololens was fun too with ET. If you have to look at a lot of letters you are going to get tired of it fast no matter how good it is. It's just not a great input method for typing compared to a real keyboard. If they are so good at hand tracking it'd be better if they put a virtual keyboard on a surface near you that you can type on (I bet they'll have that option eventually)
His video had lots of harsh criticism of many features of this headset that probably cost Apple $100MM each to build. Several parts of the demo were met with Brownlee's condescension, but one part even seemed to earn his condemnation: the part which implicitly required a dad weirdly wearing this at his child's birthday celebration so that it could be replayed in the future.