DNG can contain unmoasiced (raw) data or it can contain demoasoced data (jpeg/tiff/etc). What you’re exporting out of Photoshop is demoasiced (non raw) data. DNG is a container format, so it can hold all sorts of different formats. In the best case scenario in exporting from Photoshop, the DNG has high bit depth tiff, in the worst case it’s a jpeg.
So just cut the DNG format all together and go for a tiff, darktable will be able to read that for sure.