I'd think the art program shouldn't matter too much, as long as a desired style is achieved by reducing the size and color palette to certain levels. Keeping it open allows many people to put their entries in without needing to learn a new set of technical proficiencies
The only way I could think of 'cheating' pixel art would be to render a 3d model/scene in a miniscule size without any antialiasing or full range shading. Some game studios I've seen using that technique for their sprites, as it looks pretty close to true pixeling while having accuracy beyond handmade

this is doable in blender with a plugin and other programs I'm sure