[AgX]

...the net effect is an enlargement similar to that obtained by an extension tube of the same length as the teleconverter, but without loss of working distance.

The lenght of the extension ring (or bellows) forms an additional extension of the lens.

The lenght of a tele-converter is of no interest. What is of interest is its magnification factor. This depends on the optics of a tele-conveter, not of its length.
(Today typical factors are 1,4 and 2)