This mod simply does not work right with the hardware renderer due to all the hacks.
The main issue with the intro is that it depends on a weird combination of skyboxes and horizon effects that cannot be replicated in the hardware renderer without a lot of extra work - and this mod's intro practically being the only thing in existence to depend on it, I saw little point supporting it. Back when this was released I officially declared it unsupported due to all the render hacks that don't work right with hardware rendering. Many maps were totally unplayable due to all the window skybox hacks slowing it down to a crawl, which even now do not play nicely with how the hardware renderer processes portals.
This really could use an upgrade, with 3D floors replacing all the Transfer_Heights hacks and other things it got.
BTW, having played 007LTSD recently, that mod also could use an upgrade, although not quite as extensive as SSD. But there's also a few places where some incompatibilities have developed over the years, mostly related to the bridge things it uses throughout.
As for permissions, the text file states:
Authors may NOT use any of these levels as a base for modification or re-use.
The only exception is for fixing any bugs that appear with newer ZDoom versions,
in which case let me know and/or send over any patches etc. so I can add them in.
The main problem, of course, is that what is needed here is not bugfixing but some extensive rework. If TUD gave permission to create an updated version targeted at modern GZDoom, I'd be happy to help with it.