I always felt that stuff like Titans or Baneblades are customised and adapted designs from the Dark Age. Who's to say they're any more legitimate as actual field-deployed designs than that massive tank the Na*is were reportedly developing until they realised it couldn't move? We don't know in what capacity STCs were regarded at the time, they might not be the bibles of technological knowledge people in the 30K/40K continuum consider them to be.

There's a story, I can't remember which, where they accidentally activate a Chrono weapon on an ancient Imperial battleship, wiping out a Necron fleet as it turns them to dust. I think the Dark Age is meant to be the apex of human civilisation, the highest we could ever hope to get as the stumbling, blind species we are in the grand scheme of things. It's possible that the Warp was harnessed directly as an energy source, maybe development of a pseudo-Webway between worlds was underway. Who knows? The possibilities are endless.

I've always seen 40K as the story of what happens after every other Sci-Fi universe ended their story. Firefly, Star Trek, even Halo could all have happened in some form or other, and even as Mankind pushes towards increasing greatness in stories like Star Trek, it'll ultimately end in collapse afterwards. Sci-Fi is full of the stories of Mankind's rise to galactic power, 40K is the story of the fall from grace and how long, how drawn out the end would be. It's not in any way realistic, but it's amusing how almost every sci-fi story out there could theoretically be happening in the 40K universe, or at least, have happened in said universe.