Good luck running 70m in a CO2 dense atmosphere. And CO2 hugs the ground it does not float away. It will persist in low areas for quite a while.
Anyone in the local vicinity would need to carry emergency oxygen at all times to be able to get to a safe distance in case of rupture. Otherwise it's a death sentence, and not a particularly pleasant one as CO2 is the signal that triggers the feeling of suffocation.
It's unlikely that the thing will burst and disperse all CO2 immediately. It's just slightly higher pressure than the outside (that's the whole principle). So you have a slow leak of CO2 to the outside. You don't have to run that fast (or run at all).
The way I understood the quote, the safety distance is when they have to do an emergency deflate (e.g. due to wind). The way they calculate the 70 m is probably based on the volume and how large of a area you cover until the height is low enough that you can still breath.
Generally, because it's leaking to the outside, where there is going to be wind it will not stick around for long time I suspect.
> It's unlikely that the thing will burst and disperse all CO2 immediately.
This requires the people running this facility, and all the facilities based on it built by unrelated organizations in the future, to not cut engineering corners on the envelope. I don't take this for granted anymore. But as long as you don't get a big rip, then yeah, it'll be hard to build up a dangerous amount. I wonder if a legally mandatory cut and repair trial on the envelope would reduce risk significantly.
Speaking of wind, I also worry about whoever is downwind if there's a big release. I bet 70m is not quite far enough if it's in the wrong direction.