In general this type of protection is intended to prevent explosion by means of sparks or heat. In contrast to 'i' types, the 'mb' type implies that your pcb itself can be an ignition source, that is it potentially has enough energy to ignite the flammable components. So these requirements guarantee that in any case the potential ignition source (conductive part of your PCB) will not be able to reach explosive atmosphere. And under the requirements of the standard the epoxy of the PCB acts as an incapsulator (you don't need any additional compound). But this applies to the board itself, so your TH components probably will be encapsulated separately. I'm not sure about exact requirements (I've never sertified 'm' type), but from my practise with sertification, it's better to play safe.