nope not true. you can use your iPhone's wifi connection to tether.
I thought this was the case too, as when I tried tethering to test it, I had wifi connected to the iphone while I was tethering, but when I entered a website to find out what IP address my laptop was using, it was not my routers public address, so the phone was using the 3G gateway, and not the wifi to tether to the laptop.