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They seem very confused if they think WooCommerce is a payment gateway, wtf?
TLDR: browser developer consoles let you see the code that runs a website. Also some sellers redirect to other sites for payments.
Bit of a warning though: Stripe JS loads when the plugin is installed whether the site is configured to use it for checkout or not. Also redirecting to other sites for checkout is not in itself nefarious just that it does happen to be a technique to hide the sellers product from the payment gateway.