I was overly enthusiastic, it seems. I assumed if the tool works for Civ:BE, it would work for Civ5... but it doesn't.
Doesn't how exactly? The game claims that it has failed to read the texture (in BE, if you give the game a bad texture file, it will show a message box with an error when it tries to load it)?
What is the expected format of the input DDS? I'm feeding it 22.214.171.124 ARGB files and get files out that are at least readable in an editor... but I don't know what you intended it with, so maybe there's an issue.
Expected input is uncompressed DDS. The decompressor preserves the format of the original: copies over the header, rewrites the width/height fields, then copies pixel blocks from the compressed image into appropriate places of the decompressed image. It never tries to interpret the pixels. Compressor does the same thing in reverse.
I've tested it in Civ:BE by decompressing and then compressing again various interface textures (technology atlas most prominently, at various resolutions, as it's the biggest interface texture in the game). Try something similar with CivV - pick a few interface textures, recompress them, see if the game is still able to load them. If it is, then there's some kind of incompatibility with the input images you provided. If it isn't, then V uses a different format than BE.
I don't have CivV available at the moment, so i can't check this.
I've skimmed Dragon Unpacker source code after publishing my FPK unpacker, and from the comments there i've gathered that V used a bit different FPK format than BE (with dumb filename scrambling, for example). It's possible that FTXTInterface format also differs.