It's a mystery to me too. It's a nice thought that they included a drain which is necessary considering how poorly the cars take on gas. Very slow to fill and it easily overflows because of the design.
As you can see from the first picture of an original car there isn't a problem getting gas into the tank however is does lead to the problem shown in the GT40 crash test where the front wheels are pushed back into the tank and gas going everywhere.
Back to our issue, the next picture show the filler neck just below what's normally visible. There are three rubber lines that come up to that area, two help vent the tank as it's being filed, the other is the drain line which exits out the bottom of the car. If you look in the lower right of the picture of the open sponson you will see the other end of the tube as it exits just to the side of the gas tank (when it's there). I'm not sure what the problem might be. I haven't pulled the fiberglass side cover that runs under the door along the full lenght of the gas tank area. When they put that part on they might kink or block that end of that line. Otherwise it's a straight shot.
WARNING Will Robertson: Just so you know, the filler necks a not easy to put back in after they've been pulled out mainly due to the routing of the three rubber lines. I've included a few more pictures a couple of which show the flapper valve that keeps fuel from surging up to into filler neck. And yes you do have to take the front suspension apart to get the gas tanks out and no there wasn't much of the car I didn't take apart.