You are confusing archive.is with archive.org. Although archive.is does have an extension[1] it doesn't appear to capture any of the page contents, it just simply sends the url for archive.is to crawl.
I wasn't exactly confusing them but yeah, I did link to an archive.org article. I was having difficulty finding something specific to archive.is.
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.
1: https://chrome.google.com/webstore/detail/archive-page/gcaim...