Cyclocross is probably the perfect place to use a hidden motor as a cheat. The races are short (women's U23 race is 40 minutes) and you can change bikes. You don't need a large battery, because they will likely be using the bike for only about 15-20 minutes. And with the chaos in the pits, swapping a battery is probably doable. It won't work for all courses, but for some? Definitely something that can give you an edge.
Because of this, I find it perfectly credible that someone would do it. The fact that Van den Driessche's brother is currently suspended for doping doesn't make her look any better.
If you read the UCI rules, the fact that the bike was in her pit means she broke the rules. And the penalty includes a minimum six month suspension of her team as well. In this case, her team is the Belgian federation. Yep. The Belgian federation could end up banned from cyclocross for six months. Chew on that for a minute.