*For you. I think it's a obvious and great advice, software engineers shouldn't define themselves by their stack (Java programmer?).
> They'll have an even more difficult time finding a job if they decide to switch stacks now.
That's why planning your career on changing stacks is such a good advice: you do it before you really need. I have kept a main stack and a "side project" stack for my whole career, going from PHP, ASP, .NET, RoR, clojure, Python then JavaScript for the longest with Rust currently as the side stack I'm learning. Had I got stuck on PHP I would regret it badly.