First major wasn't during nip era, their era ended with their map streak. They were still good, but the whole idea behind the nip era (and why it's considered an era) was because of their 87-0 streak, which ended before the first major began and when fnatic started dominating.
Indeed SK era was with 2 rosters, that just cements the fact even more so (with the lack of a major as well) that they didn't have en era.
But SK and Liquid won lower tier events, but when it came to the most important title of the year (and the most prestigious one), they failed to show up. Like I said, if you want to be the best and establish an era, you have to do what no one has done before. Simply winning smaller events (even if their prize pools are large) where some teams might not attend and during roster swaps, it's simply not enough to establish an era. They had a very good run of dominance, but neither team had a fully-fledged era.
Only fnatic and Astralis, in my opinion, had their own true eras. Maybe NiP but the fact that they didn't win the major and that their 87-0 streak was done in a period of minimal competition (basically VeryGames and WW), it's hard for me to justify it. But then again, they did something no one had ever done, so I might still actually consider it an era. But definitely fnatic and Astralis are the two the stand out the most.