Trying to understand coaxial cables
Either you were not listening carefuly or your teacher was cheating. If the source impedance matches the load impedance (complex conjugate \$Z_{load}=Z^*_{source}\$) and they are connected with a coax of same characteristics impedance (\$Z_c=\sqrt{\frac{L}{C}}\$, resistance of wire and conductance of the isolator are ommited, since negligible), then there is no distortion, however the signal would be delayed, but not distorted. 
What you saw, wasn't a distortion due to capacitance of the coax, plausible cause is that signal generator reached its max capability in terms of frequency output. The generator was unable to drive a square pulse at such high frequency, that's it.