Google's John Mueller said that just because some content might be behind a paywall, it doesn't mean Google sees that content as thin content. Google allows content paywalls and will show them as much as it shows content not behind paywalls. So no, Google does not automatically treat paywalled content as thin content.
This is the tweet on Twitter:
Content behind paywalls is not automatically thin, and people can choose different ways to monetize their sites.
— 🍌 John 🍌 (@JohnMu) August 30, 2021
Why would someone think paywalled content is thin? Well, generally when you hit a piece of content that is paywalled, you may see a paragraph or two of the whole article. So maybe people think that is all that is there and that it qualifies as thin content?
Either way, word count is not used by Google:
We don't use word count.
— 🍌 John 🍌 (@JohnMu) August 30, 2021
Forum discussion at Twitter.