Just noticed that long pressing the middle button triggers google assistant. Seems the only useful thing QC35II has is better noise cancelling when using the mic in a noisy environment.
Not sure if this is related to a new feature or the latest firmware update.
The middle button has always allowed access to Siri/Google (not necessarily Google Assistant I think it depends on what android you have). The inbuilt assistant button gives much quicker access to Google assistant as well as access from non android devices (with app download). Instead of having to wait for the beep or even say "OK Google" you press the button like a walkie talkie and speak straight away. Processing of commands also seems to be much faster.