I am wary of that sort of very specific advice. Why LAMP, just because it's fashionable? There are many, many other sorts of web stack alone, let alone the many, many other sorts of application. Same with GitHub (which I use myself, btw), there are plenty of others.
It just sounds like ESR's advice, to be a great hacker you should be just like me...
Sure, just because it's fashionable. X86 is fashionable, C++, Java, JavaScript, HTML, CSS. You definitely should try to become familiar with everything fashionable in our industry. Yeah they're all messy, but they are the reality of how things are done. People who think they don't need to are why we have the term "ivory tower".
Specific advice is good when the audience doesn't necessarily know enough about what they need to differentiate between the various options. Thousands of man-hours are wasted every day by people trying to decide which of two (or more) entirely adequate tools they should use.
I think it's better presented as a heuristic instead of an imperative (as it is here), though: "Use X until you can articulate a reason not to."
It just sounds like ESR's advice, to be a great hacker you should be just like me...