I learned Android on my own time, because I liked the idea of a Linux-based phone OS. Most of the jobs I've gotten have been EXACTLY because I've been playing with the right technology on my own time. OP is right on, and VexXtreme is making noises like a mediocre developer who has a chip on his shoulder.
But The fundamental data structures/algorithms don't change and the problem solving skills don't change. For example, get an Oculus Rift dev kit, fool around on that for a few months when the consumer version comes out you'll probably have more work than you can imagine. It's the same pattern.
On the other hand, Linux kernel programming, which is what the author mentions, hasn't changed that much over the years. What the author seems to bemoan is that the developers are letting those core skills slide because they become dependent on the tools they use in their day-to-day jobs. But actually, I'm not convinced that the mediocrity of the candidates was due to decaying skills. They may not have been particularly skilled to begin with. Experience doesn't necessarily correlate to competence, after all.
Doctors are expected to keep up to date with the most recent research as part of their jobs, and they are payed to do so by going to conferences etc... I don't think the same can be said for software development.
All other things being equal, the person with experience in the specific tools the organisation uses will get the job.
Its just an unfortunate consequence of the industry we are in.
You are of course free to not chase the bleeding edge tools in your spare time, and youll probably stay employed just fine. But there is a risk you can be left behind or miss opportunities.
The people who were hacking on early IOS, Rails, Scala etc are todays experts enjoying niche markets. The people who didnt are working on VB or PHP for a Sausage factory somewhere.