Nerfing GP's ult like this is like saying Morgana's Pool shouldnt set it off.
about that...
[{quoted}](name=Fizz Enthusiast,realm=NA,application-id=3ErqAdtq,discussion-id=XhPEjuQM,comment-id=0016,timestamp=2017-01-02T09:10:55.900+0000)
##Fizz Enthusiast http://boards.na.leagueoflegends.com/en/c/gameplay-balance/XhPEjuQM-varus-thunderlords-vs-lux-thunderlords?comment=0016
When thunder lords first came out, didn't have the tech to do it. I believe we have the tech to do it soon.
The reason this occurs is that we have no way to determine spell origin. For example, on the second tick of Morgana w, thunder lords has no way of knowing if that instance of damage was from W, q, auto, r etc. As such, it has no choice but to treat it as a separate instance of damage.
This would mean that once the tech is out, any instance of damage from a spell will only refresh the thunder lords stack and not add one (morg w, mf e, etc.). Multiple cast spells will count as a separate cast for thunder lords, Eg. Ahri R. Effects that state "your attacks/spells apply X" will count as one spell including the auto attack (Eg. Teemo E). Recasts count as one (Eg. Anivia q, sejuani w). I believe aurelion passive will count as one for the game duration.
For the varus example, I'm not sure what's going on there, probably mum's linguine.
Edit: oh right, varus w is of type proc, whereas lux passive is of type spell.