I loved that quote when I was a teenager, but over time I came to hate it. We can't master everything. We don't live long enough, and besides, the list of things to master grows superlinearly, which means mastering even one thing requires specialization.

One should absolutely have a diverse set of skills and interest and knowledge, but it's also important to pick one or two things to truly master.

It doesn’t say to master all those things...in fact that’s the opposite of his point, which is to be reasonably competent at a lot of things rather than being helpless without a specialist around. Heinlein’s characters are (sometimes hilariously) self-sufficient, but respectful of true expertise.

My perspective has been to be able to "speak the language". You may not be able to rebuild your engine, perform surgery, build your own PC, etc. But know enough that you can have a meaningful conversation with the experts.