The mismatch loss between 50 and 75 ohm cable is 0.4 dB (or ~5%). It's not really significant unless you're doing small signal work (which wifi isn't) or if you're doing high powered work where 5% loss heats the coax (which wifi isn't).
Wouldn't there be a problem with reflection? Attenuation is not the issue here, reflecting even a fraction of a strong enough signal could potentially even be destructive for the device. It's possible though that WiFi devices, particularly those with coax ports, have protection against this.
