Not sure if I really believe this.
I'm someone who likes to pick up new skills for fun & fulfillment. And yet, when it to career success, roughly 80% of my net worth is attributable to two, maybe three skills: being really good at Javascript, and having solid CS fundamentals in data structures and algorithms. Together they got me into Google, where I picked up the third skill (knowing the full Search stack from the crawler to the browser), which made me very valuable to Google and got me a bunch of stock that's done quite well since.
Now, I also picked up a bunch of other skills while I was there: visual design, interaction design, product design, empathy, management & leadership, some machine-learning & data science, distributed systems, how to approach a vaguely-defined problem with no clear answer and make forward progress on it, how to balance lunch & a drink on your laptop and show up late to a meeting without spilling, etc. And I had some skills from before Google: investing (professional, not just personal; I worked for a company that wrote software for hedge funds for 18 months), game design, compiler design, Django, even some PHP from my college days. And I've learned even more since leaving Google: entrepreneurship, corporate structure, identifying markets, Android development, iOS development, watchOS development, blogging, YouTube video production, Photoshop, how cryptocurrencies work, a lot more unstructured data mining & data science, what risk actually means, etc.
But when it comes to cold, hard economic returns, nothing seems to beat identifying an in-demand skill, doubling down on it until you're noticeably better than the vast majority of people a company will interview, and finding a wealthy buyer who will pay you for it. (On a corporate level, this translates to finding a growing and potentially large market, doubling down on it until you're noticeably better than every other company in that market, and finding a wealthy buyer who will pay you for it.) I kinda wish it wasn't so, because I really enjoy the learning-new-things part of life, and I enjoy learning hot new technologies or algorithms more than obscure proprietary frameworks in a megacorp. But people pay you for the portion of your skillset that is relevant to the problem they want solved, not the portion that is personally fulfilling.