The article is about cyclo-cross racing, what has existed for ages, predating MTB'ing by several decades.
It existed as a way of racing in the wet and icy off-season, when road-racing is too dangerous or weather-dependant : ride off road round a park or field, up lumps and bumps, through mud or sand, up and down steps, over barriers or logs, etc
Races can be hard whilst being shorter than road races.
Basically you can think as a cyclo-cross race as a cross-country cycling&running race - there will be various obstructions and obstacles which cause you to have to get off the bike, chuck it on your shoulder and run with it, remount and ride a bit, get off again and run a bit more, remount and ride a bit more, etc.
A cross-country MTB race however will have a 100% rideable course (assuming you have the skills !) - there will be climbs and drops, muddy bits, etc but you should be able to ride them.