I have been working on a code base that is now 14 year old for many years (almost since the beginning), and is now well over 1M LoC of typescript (for Nodejs) - we are only 20-30 engineers working on it, rather than the 100-1000 suggested on the article. And I can say I couldn't agree more with the article.
If you have to work on such projects, there are two things to keep in mind: consistency and integration tests.
Yes. I remember working in a 700,000+ line PHP code base that around 30% unit test coverage and an unknown percentage of e2e test coverage. I kept my changes very localised because it was a minefield.
Also, the unit tests didn't do teardown so adding a new unit test required you to slot it in with assertions accounting for the state of all tests run so far.
If you have to work on such projects, there are two things to keep in mind: consistency and integration tests.