Here's some original footage. They appear to be holding the pufferfish in their teeth. Maybe this limits the amount ingested, and maybe diluted by the water.
https://www.youtube.com/watch?v=vwF2p626su4&t=131s
I used to work with tetrodotoxin. It blocks sodium ion channels causing nerves to go quiet and not function. And it doesn't take a lot. I had to be very careful weighing it out so as not to inhale crystals or get in on my skin. They used to serve wild pufferfish liver (fugu) in Japan, which is highly toxic. Prepared the right way, it brings numbness to the lips etc, but there is always the danger. Now they use farm-raised pufferfish which are much less toxic, apparently.
-d.