I think in this case it's a bit more nuanced. I don't think the Usenet denizens of old were against new members. September usually brought an influx of new users when university freshmen would start school (at the time, many people's first introduction to the internet was when they got a connection at their university). It would be a bit of work, but over the next couple months the old-timers would acclimate the newcomers to Usenet etiquette, and all would be well.

The issue with the Eternal September was that AOL users suddenly got Usenet access, and the constant flood of new, un-acclimated users was too much for the existing users to handle.

Yes, I agree that there's still a degree of gatekeeping there, but communities have standards, and community standards (assuming they're good ones) are usually a great way to maintain civility and keep things on track and on topic.