Battery heating up instead of a coil in a simple heat circuit
Well, I'm not surprised in the slightest.

"about 4 inches of bare AWG 18-gauge wire" would have a resistance of around 2mΩ and the current carrying capacity of it for chassis wiring is in the order of 16A.

So yes, the battery was short circuited.  It would be trying to draw in the order of 4500A from the battery.

The whole notion of doing that is just daft.  Whoever wrote that article doesn't have a clue.

There is a special form of wire, called nichrome (short for Nickel-Chromium) which is a resistive wire. It's used for creating heating elements, and for using high power potentiometers (known as a rheostat), that would give far superior results.