--Robert Heinlein
Changing diapers and pitching manure? Not so much.
Dying gallantly? Only get to practice that once, so how would you get good at it?
People throw this quote around because it is fun to imagine yourself as a supremely Competent Man in all these domains. Indeed, it makes for excellent fiction—-but it is just that, fiction. When you actually start trying to seriously think about investing in specialized skills like military science, this quote quickly reveals itself to have not made any distinction between life skills that the average person should have and things which require years of study.
I do agree that the invasion is a bit of an outlier, although even in that case there are questions of scale.
I've always loved that quote, but my reading is: on top of the everyday things, it's worth picking up several skills which require some deep study. No, you won't get them all, but having several -- and being open to learning something new if a good opportunity arises -- is valuable.