What part of "best effort, unsupported" do you not understand, if you've read the article?

You're underestimating the amount of goodwill-run, unstaffed projects that any big corporation accrues over time, which accidentally become load bearing without anyone realizing until something goes wrong. Such unstaffed projects are usually very stable (from not having pressure to add features or earn profit) and therefore "just work" for years until something unusual, like an accidental DDoS, happens. In that time, the original author(s) and everyone with context have left the company. This is a very hard process/human problem to solve at FAANG scale.