It’s obviously not mandatory or anything, but posting an archive link is always a nice thing to do even when the news site you’re posting isn’t paywalled or blocked. For one thing, if the article gets changed or removed, it will still be there on the archive site, in the form it was when the snapshot was taken. It also doesn’t give that site traffic when people reading your post click through to read the article, and usually that’s good, considering the profiteering of mainstream news sites is not something leftists like to contribute to. Those MSM sites also can’t track people who see the archived version instead of going to their site. And finally, although sometimes some ads will get archived along with the snapshot of the news site, generally it makes it a lot better for folks who don’t use an adblocker, as it cuts down if not eliminates intrusive ads.

So yeah, no big deal, but it can be a helpful, considerate thing to do for future reference. I’m only explaining all this because you asked the person requesting the archive link what they meant.