That's a brilliant quote
I think there is a way in which that idea appeals to someone young, because until a certain age, you can imagine being anything, knowing everything, your future is unlimited.
And then somewhere around middle age, you realize that whether you are the best specialist in a field or not, whatever you've achieved, your life is inevitably going to be a tiny fraction of the scope of possible human experience and far from the extremes of what is possible.
This is a difference of semantics. "Do a thing you are especially good at" vs. "Do two things you're really good at ... to create a unique result" is another way of saying "find a thing you're uniquely good at."
This is like in accounting/economics when someone says, "Everything is a variable cost... if you wait long enough!" Sure, the payments you make on a 10-year contract are variable if you're thinking in time periods of 100 years (in years 11-100 you can decide whether or not to keep paying for the item), but that's rarely the case.
Similarly, when people talk about specialization and "finding something you're good at", I would assert that the norm is that people think in terms of one specific area/skill. Hence, it's important to talk in these terms and point out to people that they should consider that they might actually get the most benefit from being good at two skills.
Why would you follow advice from a certified nutcase?
Dismissing advice out of hand purely based off of the source seems like a dangerous shortcut to espouse, as it's likely to lead to groupthink and echo chambers. I think, when looking for advice, one should go out of ones way to solicit different backgrounds (dare I say, a diverse set of opinions) and then synthesize them into a course of action.
See also, taking the other side to lunch.
One aspect is that being excellent at one thing is hard. Be really good at two things. And thats how you will stand out because it is the fusion of two skils that makes you unique.
First: Some advice on this by Scott Adams. https://dilbertblog.typepad.com/the_dilbert_blog/2007/07/car... I love this one.
Also a book that pretty much centers around the same point. I think the book itself is a great read. And I recommend it. https://www.amazon.com/Medici-Effect-Elephants-Epidemics-Inn...
And then there is a quote by Robert Anson Heinlein, if you want to take it to heart and live on other extreme. By no means it is a career advice but surely an interesting take.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.”