Paypal got it from eBay, which in 2000's was rolling out 20M LOC worldwide every week or two on "release trains". There, a small team of kernel engineers rotated doing the merging -- two weeks of clearcase hell when it was your turn.
And, since eBay wrote their own developer tools, you'd have to deploy different tooling depending on the branch you were on. But because of their custom tooling, if there was a problem in the UI, in debug mode you could select an element in the browser UI and navigate to the java class in a particular component and branch that produced that element.
The angle we took in the blog post focused on what was widely documented and accessible to the community (open-source tools like Bors, Homu, Bulldozer, Zuul, etc.), because those left a public footprint that other teams could adopt or build on.
It's a great reminder that many companies were solving the "keep main green" problem in parallel (some with pretty sophisticated tooling), even if it didn't make it into OSS or blog posts at the time.