How could google index a page if the page requires an account to view? I'm pretty sure googlebot has no such account. Surely such pages are invisible to google.
Somewhat related, the hidden answers on Quora are always visible/indexable in the page source (on occasion that's how I've read answers without signing in and without downloading an extension)
Many forums give read-only access to Google and other search engines so that they can be indexed without requiring an account, sometimes by UA or IP range.
It's called cloaking when a bot gets one version of the site and a normal user gets a different page. This can be done based on user agent or IP.
Then it might be possible to bypass paywalls by reading from google's cached version.