It's a gas in an open space, it diffuses very quickly.

Yep. When I had to fill CO2 tanks at a paintball shop yes there were times that I had to open a door (I mean we were talking a lot of fills in short time, btw fills had to start with draining the tank's existing volume so I could zero out the scale) but even indoors a door+fan was enough to keep even the nastiest of sale days OSHA compliant.

Also a 'puncture' is very different from the gasbag mysteriously vanishing from existence; My only other thought is that in cold regions (I saw wisconsin mentioned in the article) CO2 does not diffuse quite as fast and sometimes visibly so...

https://en.wikipedia.org/wiki/Limnic_eruption

I don't know the safety limits for this quantity, I hope the "70 meters" claim was by someone who modelled it carefully rather than a gut check.