I think I lived through a different 90’s than the article talks about. No nirvana, no soundgarden, no STP, no alice in chains. I did see a single Pearl Jam song listed though.
I checked the first part linked at bottom, and apparently their version of 60s doesn't have The Beatles. They had The Doors, though, so it's not like they ignore all alternative-ish rock. Just almost all of it.
On one of their other pages they have a note that The Beatles is a notable artist that isn't in Spotify (or wasn't, at the time), so that is probably why.
Have you checked how popular these artists really were at the time outside your group of friends? None of them have ever had a top-five single in the US. Several of them have very barely had a top-hundred single!