Still less risky than free-solo climbing in the mountains…
I don’t think that these two can be compared directly. Free solo climbing means that the climber matches the difficulty of the route he is climbing to his skills. He can train and check those skills in a safe environment. It is like accepting a croswind landing when you know you have done it before. The biggest remaining uncertainty is rockfall for the climber or the unexpected very strong gust for the croswind landing.
But gliding into clouds about which you know nothing and that might reach all the way down to the surface (provided they did not talk, as you suggest, to some trustworthy person on the ground) means gambling with your life. The outcome does not depend on your training and your skills in the least, but whether there is some clear air between the rocks or not. I don’t think that one has to be a pilot to call this kind of ”risk management” stupid 
Uhm… that’s Annecy, so those are more like bumps than real mountains :-). They could easily check weather down low by getting the Annecy Metars (if they were so inclined). Also when they “punched under the cloud base” therê’s actually a massive blue hole in the clouds just in front of them / above them… so I think the cloud base wasn’t that thick to begin with ^^
Uhm… that’s Annecy, so those are more like bumps than real mountains :-).
The same guys have done similar stuff in the Himalayas:
I don’t think you can call those bumps 
Even though it feels crazy scary, I must say it looks amazing.
I would wonder how they could deal with cumulus granitus.
