Not that the whole "what defines progressive music" conversation hasn't been done to death. But I did read a very adequate description that I think resolves the issue pretty well:
When it comes to the word Prog, or the word Progressive, if you're talking about innovation in music, doing something new, then Progressive is an adjective, but if you're talking about the genre, and all the elements that it has established since the 70s with bands like King Crimson, Rush, and Genesis, then Progressive becomes a noun, just like Thrash, or Glam, or any other noun that precedes a subgenre of music. And it makes sense, when people say, "This music isn't really progressive," they're thinking about the definition of the word, which is an adjective, that actively describes the music. But as a genre, usually music isn't described with adjectives. It's not Dying Metal, or Killing Metal, it's Death Metal, that sort of thing.
That's why I think, when people actually make the distinction between "Prog" and "Progressive", and someone says, "But Prog is just short for Progressive, it's still progressive," neither is wrong, it's just that, the way the term is used, it's between a noun and an adjective. And DT has been both, they've always stayed Progressive as a noun, using the elements that have been established by those classic bands like Rush and Yes, but they've also innovated quite a lot throughout the years. And maybe they don't innovate as much these days, but as a noun, they're still very much Progressive Metal.