Usually I would agree with you, particularly when we’re waiting for an arbitrary advancement. But some advancements follow somewhat predictable curves – Moore’s Law was, for the longest time, the best example. Everyone knew Moore’s Law would have a termination point, as the size of the transistor approached atomic scale. But for decades it just sort of bounded along.

In the case of fusion reactors, it is contingent on a few advancements which are largely predictable.

Lasers have been improving in a largely predictable way, both in terms of energy efficiency, but also energy density you can pump through them before they melt, etc. Like Moore’s Law, there are theoretical limits that will be approached.

Another example is the manufacturing processes required for the weirdly shaped components. Ever look at the https://en.wikipedia.org/wiki/Wendelstein_7-X – the shapes of magnets they need to make for designs like this basically require advances in robotics and material manufacturering to make it scale. These advances are somewhat predictable.

One advancement that would make a huge difference that isn’t predictable is cryogenic cooling for those same magnets. High temperature superconductors would help, but none of them can be manufactured large enough (yet). And we don’t know what potential superconductors can be discovered. So, you cannot rely on this for making your timeline.

So, yes, pulling it out of their ass, but using best guess for the trajectory of developing the technologies required. Kind of the point of Futurism, actually ;)