You don't really have to look at interstellar civilizations to make that extrapolation, you can look at our own. We "synthesize needed substances from raw elements". But what you seem to be focusing on is not the important part of his statement. There are so many resources available in a typical Sol-type system it's mind-boggling... enough minerals and water to supply hundreds of billions of people. If you have the need to expand beyond your own planet for minerals and water, and you have the technology to send a fleet of ships across light years and to other star systems to get them, you'd have the technology to do the same thing at home. Even if they had to come to our system to access such resources, it certainly wouldn't be worth the hassle to bother us here in the inner solar system when all the good stuff (that would be most energy efficient to remove from the solar system) is from the orbits of the asteroid belt to Jupiter, Saturn and beyond. Unless they were building an enclosed Dyson Sphere 10 miles thick and had to support a population in the trillions, the resources available in their home system should suffice quite nicely for a very long time.
Oh, I agree you can easily make that extrapolation. My point, however, is that it *is* an extrapolation, not a "given." It's in the nature of science fiction to extrapolate. And while some extrapolations are going to be more scientifically sound than others, that doesn't mean that stories which incorporate less plausible ideas ought to be dismissed out of hand. So while it is more likely
that an advanced civilization would stop elsewhere for water, even raw materials, it's not impossible
that a civilization that needed not just water, but food, and an inhabited, industrialized planet as a base of operations for an imperialistic agenda (all of which were stated reasons for the arrival of the Visitors in the original miniseries) would choose Earth as a place of conquest.
To be dismissive of the story on that basis alone seems, to me, an overly stringent standard for science fiction stories.