The notes were already encoded as frequency and duration. The Huffman coding was required to get that down to an even smaller size. RLE might have helped for r

The notes were already encoded as frequency and duration. The Huffman coding was required to get that down to an even smaller size. RLE might have helped for repeated notes - but I guess Huffman coding kinda compensates for that anyway, in the repeated notes will probably end up with shorter bit strings.