He means to say that if you and another person were to hold a piece of string taught over a large distance, than one of two things would happen:
1: FE: The String would be just that, taught. It would not contact the ground in between the two people.
2: RE: The string would be submerged in the body of water between the two people due to the curvature of the Earth.
Now, this is a very good point to make, and it seems that it would work very well.
However, all FE'ers would say that the string itself was not taught, but was bending in the middle, as to create the submersion. Even if you used a material that could not bend, they would deny this experiment.
So, good experiment, but wouldn't be accepted by most FE'ers here.
PS. When I mean most, I mean Tom. THinking on it, the number of FE'ers here would qualify Tom as most...