From what I've seen... if music has any detectable trace of death metal in it, it is always lumped into some category of death metal... the same works with Black metal... even when it's bred with something else, black is always stuck somewhere in the title.
Death metal just seems to be the "second word" most of the time...
The way I see it is that Progressive Death metal can mean two things. The band can be playing death metal with progressive elements, or the band can be playing progressive metal with death metal elements... no matter how they balance.