It was less of an immediate safety concern and more a case of trying to limit the variables. They were only doing tests under ideal conditions, rather than testing in all conditions. I imagine the plan involves testing in all conditions eventually, but they need to fine-tune their equipment in ideal conditions to make sure they know what "good" and "bad" look like before potentially feeding some atmospheric action that they didn't anticipate.
I actually worked in the lab with Louis' team this past summer (completely unrelated projects.)
If the wind was going faster than a couple miles per hour, they couldn't even turn their prototype on to run their tests. I think the biggest problem they're having with going big-picture with this is doing the due-diligence to prove it's not a tornado machine.
The actual presentation from 24C3 is available at https://www.youtube.com/watch?v=8lywXCZogsg