I could be wrong, but I think they were trying to solder together to have the old cable thread the new one through under barely any tension. I've done this on sailboats to replace the halyard, the line running to the top of the mast to raise the sail, without having to drop the mast to do this; You can't use a knot, it won't fit through the masthead block (pulley). I tape the two line ends together end-to-end, then take some monofilament fishing line and spiral it across in both directions, and that works like a Chinese Finger Trap, holding the two together, pulling the new halyard up and through the masthead block. Some just connect a very thin threading line to the old halyard in parallel for a couple inches, pull that through, then do the same with the new halyard.

If I did need to solder two cables together, it would not be end-to end, but "enmeshing" the two ends together, sprialing into each other, clean with solvent, and then solder. However this will make that section inflexible, which might be an issue with threading if not a straight path.