to make bulky splices as shown in Step 11 is by lining up all the conductors side by side as a bundle and then tightly wrapping the whole thing (6-7 turns) with a length of either 20 or 22 guage bare wire. It pulls everthing together nicely and makes soldering easier since the individual wires don't move while the soldering iron gets everything toasty warm.
That's one of the factors the caused the cold solder joint. It would be worth your time to go back and fix that issue (as others have pointed out below).