I feel like the examples of log4j and ua-parser aren't that great, because it would be relatively easy for any other similar lib to take their place, as it's mostly straightforward to implement, even though it still takes time.
But there are some things like Kafka, PostgressSQL, Spring Boot, Tomcat, Apache Math, ZooKeeper, the OpenJDK, and all that which are definitely non-trivial and a huge amount of time and effort, and you couldn't just take an extra month or two and have a dev on your team implement a replacement, unlike log4j and ua-parser.
I think those would be better example to discuss, and my impression has been that those things often have a company behind them offering support or offering them as a service that in some ways pays for some real devs to contribute to them, but maybe I'm mistaken.
Like for example, the author mentions working on the GO team at Google, and Go I would consider one of those big open source projects that truly are foundational and would be non-trivial and huge effort to replace. So that shows that the really big pieces do have companies hired and paid staff behind them.