Author here. You really don't want any disconnect between the rider and the uni. You're making tons of constant micro-adjustments, and any lag or reduced feedback would make riding really difficult. Suspension would also allow the wheel to move upward relative to the frame and pedals when hitting a bump.

As well as this, it'd waste energy, which would tire the rider faster and eat hops. There is also the case that most suspension solutions aren't really designed to handle the sort of aggressive lateral torsional stresses of the entire weight of the rider and their movement in the way that unicycles dish it out.

As someone who goes along gravel paths with relative frequency on their unicycle, it isn't too bad. I'm not sure if the dirt bike-style unicycle you're referring to was an electric one, but most municycles (mountain unicycles) have decently thick tyres to handle some of the jolts. Take the model I have as an example: https://www.krisholm.com/en/gear/unicycle/kh27.5

Thanks for the comment. I'll make an update to provide a tad more information on suspension.