As others said, they don't really get credit for pioneering Progressive Rock, but rather for pioneering Progressive Metal - blending the 2 genres and complementing it in different ways. Personal opinion ahoy: WDADU sounds very much like what you just described, "Yes/Ruch kind of Prog Rock with heavy guitars", but even that one has a few moments (or whole songs, like Afterlife) that wouldn't normally come from a pure Prog Rock band, and they definitely added a new layer of technical showmanship to the genre.
Images and Words, again, to me, is the definition of what Dream Theater gets credit for. This is in many ways a late 80s Glam Metal album, but it blends perfectly with instrumental breaks, time signature changes and irregularity, and while it has the distinct sound that makes me love Scorpions or Iron Maiden for example, it also never stops surprising the listener (well, on the first 30 listens or so, then it wears off).
Following albums have done the same thing, always slightly changing their influences from both Metal and Prog Rock and blending them in beautiful ways. The Mirror sounds like something that would come from early Metallica or some heavier band than that, yet they play on that riff and tear it apart and tackle it in different ways like a Prog Rock normally does with much more mellow music. The same happens in certain sections of 6:00, Caught in a Web, etc. Voices was one of the earliest examples (along with Take the Time and Metropolis I guess) where besides blending two genres into a single piece and playing with individual riffs, they also mixed completely different styles in 2 or more sections of the same song and challenged themselves to transition between them as well as they could (which they could beautifully, big time).
I am no musician so my opinion might be worthless, but to me DT was the first to play around with 2 rather distant genres and see how they could bring them together in harmony. For the first half a dozen albums they basically took different sub-genres of each big inspiration and played "Will it blend?" with themselves, creating beautiful ideas and songs. This kickstarted both the kind of distinct sound of them that so many bands try to directly imitate, but also the whole principle of bringing these two worlds in harmony - so that other bands could take on different subgenres, the way Muse takes the experimental nature of Prog and Metal and creates an even bigger beautiful mess, or how Opeth brought that style together with death metal and brought them to complement each other in ways that people other than myself could possibly appreciate and like.