It is. But the mechanism by which they start, and the paths between "energy stored on a surface" and "discharge in the gap" vary.
If the discharge takes place between insulators, then it's of the non-spark type. In these cases we understand some physical limitations to its maximum energy and can make engineering decisions based on this. Energy in a non-spark discharge is limited.
If it's a conductor, then _because_ it's a conductor the charge can flow around it and thus charge from a very large area can be delivered to one small point. Energy in a spark-type discharge is not limited by the materials of the duct (until you know the capacity of the conductor). These can be _much_ bigger discharges than the brush discharges.
For woodworking dust collectors, we know the energy needed to ignite the mixture and we can show that this is always more than is available from a propagating brush discharge.
For the case of an insulating duct with a metal pipe-joiner flange, then there have been industrial accidents where flammable vapour explosions were caused by spark-type discharges from this flange acting as a capacitor. These were in systems designed to be safe for non-spark discharges - the ignition energy was above that of a brush discharge, below that for possible sparks. This is the case were earthing is useful; it's necessary, and it's effective.