I stand corrected. P. puffers are toxic. But like most toxic animals in the sea, the bulk of the poisons is a direct result of food consumed. The following website is pretty interesting.
http://www.chemsoc.org/exemplarchem/entries/2004/icl_Quek/intro.htm
Another reason to explain why the eel was still alive (and I found other sites where honeycombs eating lionfish and porcupine puffers in other people's tanks) is that most tank-raised fish (not necessarily tank bred) do not have as potent of a toxin as wild fish. Puffers actually produce TTX, but certain foods might provide the necessary compounds needed to manufacture TTX. Other poisonous animals (triggers and others) obtain their chemical protection as a direct result of eating certain foods. In either case, these are almost always lacking in the aquarium, and thus, their poisons should be less powerful, though I still wouldn't eat a captive one
I just wonder if Eels will feed on P. Puffers in the wild, and if they survive. Something must eat these guys and be able to survive.