The point would still stand though that the total revenue from Apple Developer subscriptions would not be insignificant (perhaps insignificant to Apple’s overall revenue, but still enough to be a sizable startup’s budget on its own), which I should have noted in the original article.
Their documentation suggests otherwise[1] (scroll down to “Benefits and Resources”). “Safari Extensions distribution” and “Software distribution outside the Mac App Store” are listed on the column of the 99 USD fee.
[1]: https://developer.apple.com/support/compare-memberships/
Just remembered that Safari now only lets you install extensions from the App Store, so that would actually make sense if a paid developer account is now required.
There’s yet another reason why Safari has so few extensions.
Keep in mind that the term "registered developer" is fairly ambiguous. Does it include non-paying developers? Does it include all developers under a single business account (e.g. at one of my jobs, I was a registered developer under a broader organizational account)?