He’s attached by his feet. But a fall might still kill him even in that case. I saw a lady who was slacklining over a gorge - she fell like four times! But she kept getting back up and finally finished.
I saw he was wearing a harness, but couldn’t figure out how he could attach a safety line except for maybe getting in or out of the basket. I didn’t see him tethered to the slack line but maybe he had a way to hide it?