Hi, I highly doubt booking.com's success came from CTA button colors - or any testing on the consumer side of the platform, for that matter. Their success is rooted in how they are able to take advantage of hotels and their distribution power.
Because they test a lot doesn't mean it's critical to their success. That's a cargo cult fallacy.
Your article points to 2-3x industry average conversion rates (for existing traffic), but says nothing about the more important factors of acquisition or inventory.
Except that Booking actually has data to back this all up. It's very common to do "holdbacks" so that you, say, give an old version of your website to a very small portion of your user's, then compare its conversion rate to a version of your site that has all the latest A/B test winners. Booking also tracks religiously how conversion rates change over time.
The whole point being that the way Booking (and other) companies act is the exact opposite of a cargo cult. Everything needs to be backed up with data and challenging conclusions is part of the culture.
All that said, a common complaint of Booking is that it has a ton of "dark UI" patterns, so it will be interesting if there is any long term blowback against short term A/B winners that erode the goodwill to the brand over time.
