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I don't think there's an unquestionable way to know if a site is made in WordPress, although you can intuit by some clues: header, meta tag, robots.txt, cookie login, sitemap ...

also headless website using WP as BE only

The article says that they also look for "wp-admin" et al in robots.txt

I suspect (hope) that means none of the WordPress sites I maintain will be in the tally.

Customizing the metadata, stripping out unnecessary cruft, and moving the user and admin login pages behind IP-restricted firewalls (when possible) are among the first things I do.

you can compare it to this: https://www.isitwp.com/

It doesn't work, I introduced the url of my page (programadorwebvalencia.com) made in Jekyll (a static site) and it tells me that it is WordPress.