This is an interesting point. Indeed, in original fairy tales the way of motion usually is not specified explicitly. Phrase like "едет в ступе, пестом погоняет" are used normally, that can denote either flying or sliding or whatever. Along with Christianity we got an idea about witches flying on brooms to their sabbaths, and that can be confused with the original pagan Баба-Яга. Later, in the modern time we had several animated cartoons about Баба-Яга flying in the mortar so the idea of flying is common now. But to find out what was the original vehicle principle, the extensive research is required.