They (archive.is) would have built the extension to send the current page content to their servers and the user would have installed it so they can archive internet pages. https://help.archive.org/help/save-pages-in-the-wayback-mach... (item 2)
1: https://chrome.google.com/webstore/detail/archive-page/gcaim...
I think the distinction between the two is moot in this post. The question could very well have been "How does archive.org bypass paywalls?" Though it's interesting that archive.is seems to just crawl the URL. Indeed that means they wouldn't necessarily be able to bypass the paywall.