I think "maximum extensibility" is not the philosophy of Emacs. Remember how long it took to have FFI support in Emacs (it was discussed in the article). Also, even with FFI, it is not possible to load non-free libraries. I think this shows some discrepancy from the philosophy to achieve "maximum extensibility". I would say GPL does not allow "maximum extensibility".

So the next question is: does the philosophy the author speaks of matches with the one of GPL, or rather, the one FSF and Stallman have in mind? Actually, I think these philosophies are different. If I remember correctly, in Emacs mailing list sometime ago, Stallman rejected idea of adding function to gcc to dump parse tree (or something similar) to use gcc from other software (I can't find the link right now). This was because non-free software could use gcc via CLI and "steal" the data from free software. Now, although it is the other way around, Emacs can use non-free external program. It matches with the philosophy by Matsuyama but I doubt that Stallman says it is the philosophy of Emacs.

I would say this philosophy is for author of Emacs Lisp program (though this was not what Matsuyama was talking about). You have many options. You could choose "maximum extensibility" but actually it is not doable due to GPL. You can follow FSF's philosophy (I am not sure what is it, but I believe there is). Somewhere in between, there is philosophy proposed by Matsuyama and it reflects current situation of Emacs very well, and I like it.