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The author of the article says this: -


  Condenser Mic basically converts sound energy into electrical energy


And this is not true - it's true for a moving coil or moving armature microphone but electrets/condensors require power to produce the electrical energy.

If R4 is made high enough in value then it's possible that the JFET in the microphone (I'm assuming that the microphone is an electret type and not a true condenser despite what the author says) is starved of current and is basically unresponsive to sound below a certain threshold. Notice how the guy "hits" the microphone to trigger it. This, in normal operation would produce a signal of many tens of milli volts and will be enough (as demonstrated in the video) to turn on the transistor Q1 briefly. A different microphone might not work at all in my opinion so I conclude that the circuit is flaky.

On the other hand many different types of microphones might work with possible changes to the value of R4 - I've never seen any information in a microphone's data sheet that can tell you the starvation point of an electret so it's guesswork.

Because good EEs tend to want to see hard claims about this or that in a data sheet, not having the information means uncertainty and unreliability creeps in - it may work first time but fail to work as it warms up or cools down. It might need R4 adjusting for a different (but the same part number) of microphone. If you made ten then some might be really sensitive whilst others don't work.