Trying to add an extra cog
You need a rear hub that can have a cog on both sides.

Some fixed gear hubs have threads on both sides, or a fixed thread on one side and freewheel thread on the other. The author apparently has one of these.

The multi-gear option is to use a disc hub and cog that has been drilled to fit 6-bolt disc brake mount. This trick is usually used to make fixed gear hubs out of MTB front hubs, but could also work here.