That looks like an indexed painting, to me, using dither brushes. A lot of 90's paint apps were geared towards stuff like this, sort of toolheavy. Not that I'm an expert, or anything, but 256 colours seems like more than you'd use if you were straight up pixelling something that's mostly monochrome like this. It's still impressive, but it's more like a progenitor to the sort of painting people do in photoshop nowadays than actual pixel art. I think.