DNG has been expanding since it was first introduced. Put simply, DNGs can be the original type which is just like the raw file and contains all the raw data (this is what your Pentax creates), or a later type of lossy or compressed DNG which is much smaller because the raw data is demosaised. The editor should be able to use them for those types of adjustments and, when applied to your raw files by importing the xmp files, they should be almost optimal. The "almost" is because you should review the results carefully to reassure yourself, but any risk is as much about the skills of the editor. So you can try their service and see how close they get.
If other editors don't accept DNGs, maybe it's because they don't use Adobe software or they don't know how to get the adjustments from DNGs back to you so you can apply them to your raw files.