SD Card Trace Inductance and Capacitance
Trace inductance is series and the capacitance is parallel and, for an un-terminated line will produce a 2nd order low pass filter that will slow down the data edges. I'm going to take a liberty with the math here and assume that the inductance and capacitance are just two lumps - these will produce a cut-off (or natural resonant frequency) of: -

f = \$\dfrac{1}{2\pi\sqrt{LC}}\$ = 199 MHz (using 16nH and 40 pF)

This sounds conveniently higher than 20 MHz by a factor of ten so it's probably one of those "committee" numbers that allows for the possibility of an extra little bit of rise or fall-time edge deterioration due to the PCB traces and ground plane.

A quick simulations shows that the edge rises and falls will be extended by about 3.7 ns from the 90% point to the 10% point: -



I have assumed that the series resistance of the driver is 50 ohm (in series with the inductance of 16nH). It could be that your driver (50 MHz) has better characteristics and can deliver a little bit of controlled overshoot at the load and thus get significantly faster rise/fall times.

Another quick check shows me that if the driver output resistance is 33 ohms the rise and fall times are about 2ns. If the driver output impedance drops to 10 ohms the rise time and fall time drops to about 1ns but there is significant overshoot (about 40%) and this is probably unacceptable.

I believe that the 16 nH and 40 pF limist are committee values to manage rise/fall times and overshoot. They probably work in conjunction with other requirements (maybe driver output impedance?).