When signals leave FPGA boards, it is fairly common to have 330 Ohm series resistors to protect the device. This sometimes shows up in uC boards as well.
So, I wouldn't spend the least bit of time worrying about a 6.5 Ohm resistance. If you want to do the math, find out what the input current is on the SPI/MMC devices and compute the voltage drop across the resistance. Then compare the result with the switching threshold voltage. You can do all this and still come up with the fact that 6.5 Ohms is insignificant.
What I don't understand is how the turn-on and turn-off times can be so slow (on the order of 800 ns each) and the device still has a -3dB frequency of 300 MHz. I'd need to think about that. Even so, it might not matter.