Don't listen to the never happy professor Grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrumpy...
Instructable is a nice initiative. For sure somes projects are really basic, not pro.
But that is exactly the principle. Things done by amateurs.
and some are amazing...
So about the project mentioned above, just give it a test, and you will learn a lot.
You can buy a guitar tuner for less than 9€, so I suppose it is for that reason you want to do a guitar tuner yourself ?
The range allowed by the Arduino Audio frequency library spans from 60 to 1500 Hz.
Guitar string frequencies are (standard tuning) :
329.63 Hz (E),
246.94 Hz (B),
196.00 Hz (G),
146.83 Hz (D),
110.00 Hz (A)
82.41 Hz (E)
That should do it.