Yes, it is an assumption, but I think it is a well-founded one. The Jedi obviously associate Force use by people they think are evil with the Sith. In Ep. I Qui-Gon assumes Darth Maul is a Sith based on much, much less, does he not? I'm sure Jedi lore includes "unnatural" powers the Sith have used throughout history and since Force-lightning seems to be basic for a Sith (and among the "third party" Force-users in TCW none of them use Force-lightning) it seems reasonable they would associate it with them. You have a better point OU, but IU we know Jedi assume evil Force use = Sith.