For WSJ at least, it appears that archive.is is fetching the AMP page, which returns the full content of the article and is hidden with CSS, and modifying the page to unhide the paywalled content + hide ads.
It might be using other techniques as well for bypassing paywalls, be it referer/user-agent spoofing (some old archives of sites that echo back HTTP request headers have archive.is sending a Referer of google.co.uk).
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For WSJ at least, it appears that archive.is is fetching the AMP page, which returns the full content of the article and is hidden with CSS, and modifying the page to unhide the paywalled content + hide ads.
It might be using other techniques as well for bypassing paywalls, be it referer/user-agent spoofing (some old archives of sites that echo back HTTP request headers have archive.is sending a Referer of google.co.uk).