Q:

How can 50MHz noise impact WIFI sensitivity?

From the Murata website (http://www.murata.com/en-us/products/emc/emifil/casestudy/solution/wlan,) it mentions "it is the higher harmonic waves of the CLK signal (50 MHz) transmitted to the Wi-Fi module that creates the problem."  
How can 50MHz harmonics affect 2.4GHz?  
They are very far apart.

A:

How can 50MHz harmonics affect 2.4GHz? They are very far apart though
The fact that the frequencies are "very far apart" is irrelevant.
What matters is how many harmonics the lower frequency has and that depends on it's waveform. If it's an ideal sinewave it will only contain 50 MHz. But if it's an ideal squarewave, in theory it contains an infinite amount of harmonics. Yes infinite !
In practice signals always have a limited bandwidth and for a squarewave this is related to the rise and fall times of the wave. If the squarewave contains very fast rise and fall rates (short rise and fall times) it will have many harmonics.
If in your example the 50 MHz squarewave has rise and fall times in the order of nano seconds then the frequency components will also be present up until the Gigahertz range.
If you would use the same circuits (so same rise and fall times) and instead of 50 MHz you'd let it generate a 1 Hz squarewave, the rise and fall times would still be the same as for the 50 MHz case. So the circuit would still generate harmonics in the 2.4 GHz band. Even though the frequency of the original square wave is 50 million times lower !
This can be explained by mathematics if you consider that a fast rise/fall of a signal is like a step response, then the fourier transformation tells us that such a signal consists of an infinite number of frequency components.
I once worked on a product with a DCDC converter which switched fast (to be efficient) at 600 kHz. This product also had a receiver which had trouble at the 283 rd harmonic (169.8 MHz) of this switching frequency !
In your case you only need the 48th harmonic of the 50 MHz to be in band.

