As to why he chose medium over his existing blogs only he can tell you. My guess would be that he is using Medium to reach a bigger audience.
The confusing thing is when he he says we and us (talking about his team) it's confusing since it's on Medium. This really should be up on the Google dev blogs if it's official.
As well as several blog like posts at google plus https://plus.google.com/+AddyOsmani
There are numerous official google blogs like the chromium blog https://blog.chromium.org/
As to why he chose medium over his existing blogs only he can tell you. My guess would be that he is using Medium to reach a bigger audience.
The confusing thing is when he he says we and us (talking about his team) it's confusing since it's on Medium. This really should be up on the Google dev blogs if it's official.