I think Weldon was joking around

But the 1500+ page script was the original, full, 9 chapters of design. And as we've made clear, a good 2/3rds of that at least was cut when we decided to bring it down to 5 episodes covering approx. the first 3 of those chapters. So, as we've also made clear, that meant not just cuts but re-writes.
As Stephen said, we then also combined what would've been Episode 4 + 5 into Episode 4, and wrote a new Episode 5 from scratch (the
general idea of what happens in Episode 5 existed already, but the script was new, the puzzles were changed, and the plot presented in it was also greatly changed). It's also now being brought into a new engine, Unity, where all the programming etc will happen, rather than Torque which we used previously.