Maybe Java/PHP isn't the best example, since they are not so different.
I think it is quite possible for an experienced dev to switch from to Java to PHP and vice-versa within a few weeks.
Especially if you make use of code-reviews or pair-programming.
Let's say you are looking for experience with Machine Learning, Distributed Systems, Databases, Kernel, Graphics, Embedded, etc.
IMHO those things are harder to learn than a new programming language or framework (at least after having some experience).
That said, I think this article was more about requirements such as: "at least 3 years experience with X".
Where X is just another language/framework/tool.
It artificially limits the number of applicants and you might miss some pretty good devs, just because they don't try to apply since either they've only worked with X for one year yet or they've only used tool Y instead of X.
Let's say you are looking for experience with Machine Learning, Distributed Systems, Databases, Kernel, Graphics, Embedded, etc. IMHO those things are harder to learn than a new programming language or framework (at least after having some experience).
That said, I think this article was more about requirements such as: "at least 3 years experience with X". Where X is just another language/framework/tool. It artificially limits the number of applicants and you might miss some pretty good devs, just because they don't try to apply since either they've only worked with X for one year yet or they've only used tool Y instead of X.