Did you read the description at all? You can only dismiss warnings not alarms. You cannot dismiss life threatening situations.
"You can wave your hand in front of Protect to dismiss the beeping — essentially, like pressing snooze on an alarm. As long as the situation doesn’t worsen, then your smoke alarms will never sound."
"For severe issues, Protect may opt to bypass the "heads up" warning and simply sound the alarm"
Still it doesn't say that this hand wave feature will stop working in these severe situations.
Ofcourse, I am sure they will iron things like this out before shipping.
It seems that the Nest will specifically tell you with words that it has detected CO, so you'll listen.
I wouldn't be surprised if the CO detection can'd be dismissed since you would never really want to dismiss it (unlike smoke).
At first, it seems like an awesome feature when you know the source of fire. e.g your kitchen.
But consider CO alarm cases. Top 10 reviews in Amazon for a Smoke alarm + CO detector talk about how CO alarm saved their lives. Most of the reviewers at first thought it was a false alarm. If it was Nest they would have "easily" dismissed it and probably died.
But that said, other features like motion detector, night light, syncing with Nest thermostat to shutoff the heater, cool design, make it a good buy. As long as I don't dismiss all alarms by default.