Version 2.7 opens the a6400 arw files fine. You could try that.
It’s a bit unfortunate that dt doesn’t use dcraw to open unsupported raw files, like RT can. Let’s take Sony a6600 as an example: I can use ExifTool to find out the correct raw black / white levels, I can set the WB myself, I can set the input profile myself, but still dt doesn’t let me open the files from that camera, whereas RT doesn’t have a problem with them.