Not much different than the old flat-top "hammock" aerial idea used by AM broadcasters in the 20's and early 30's except it only has two wires and "fed" at the end. As was mentioned above, adding extra wires effectively increases conductor cross section, increasing capacitive end-effect therefore increasing bandwidth, if I'm not mistaken. Also, the larger the conductor diameter, the lower the feedpoint Z is going to be at the ends, so that 1000-2500 Ohm figure will vary depending on the size of the conductor. The whole idea of the folded dipole is that the center can be series fed with typical balanced line that has a higher Z0 (300-600 Ohm) - hence a more appropriate feed point Z for balanced line. See "cage dipole" in Orr's Radio Handbook or ARRL Antenna Manual.