The capability was indeed there - but the various tricks and optimisations used in this game are very much "building on the shoulders of giants".

That is what made demoscene cool, surprising others with demos that in theory were impossible on existing hardware.

Marble Madness was a thing at the time. [1] There's a ZX Spectrum port, that's awful, but it maintained the same 3d goodness.

[1] - https://www.youtube.com/watch?v=zByAew6AKJ8

Yeah, with those early machines it was inevitably about lookup tables, and producing the illusion of doing work which was otherwise impossible in the number of cpu cycles you had available. I did lots of this sort of thing for the C64, where similarly CPU cycles were short, and then you end up scratching around trying to find enough spare memory...

It's great to see people still exploring and pushing the boundaries of what these machines are capable of. Coding this on a more modern machine, getting the effect right, then back-porting to the Spectrum seems like a smart move and would have been a game changer during the prime of these machines!

I doubt that. It takes more or less 100% CPU on the scanlines affected.