Boulder has a city population of about 100,000 and a metro region population of about 300,000. Here are some random Street View locations from European towns and cities of a roughly similar size.
Reading, England: https://goo.gl/maps/yQaTTaWQJav
Bonn, Germany: https://goo.gl/maps/Pu9Ro28F4UH2
Caen, France: https://goo.gl/maps/ppgL2J8nsAJ2
Vicenza, Italy: https://goo.gl/maps/rMtCBEqC7vr
Click around a bit, explore the neighbourhood, get a sense for the place. Get the picture?
American urban development happens on a scale that is simply incomprehensible to most Europeans. I spent a good ten minutes clicking around Boulder, looking for the city centre; it took me that long to realise that there wasn't one, at least not in the sense that I understand.
Of course, you wouldn't want to live in Boulder without a car. Pretty much the whole point of being there is to get into the mountains.
So yes, Denver has a few walkable areas that are nice. But if you can only walk 80% of the time and still have to own a car for the other 20%, which has to sit in a parking spot somewhere all the time, then we're back to the subject of this article.
1. I'm not saying it can't be done, but if you have kids, or aren't able-bodied, or don't want to spend 3x - 5x more time on public transit than you would in a car, or live in an area with extreme weather conditions, or don't want to be weird-sweaty-bike-person all the time, etc, then you need a car in most of America.