Highlight-recovery is not a strength of darktable (yet). You can play with filmic reconstruct and/or try highlight reconstruction in LCh or color mode. I have also used the color reconstruction module with some success. But in this image some parts are massively clipped and the software can only “guess” the color from the surroundings or display the texture in grayscale. If you look carefully you see also some artefacts in the nice RT version of @marter .
But there are some ideas to improve highlight reconstruction in darktable. See for example here and here.