Please note: This is a simplification for beginners. It is actually much more complex than this but you don’t need to understand all the science behind the scenes to use ISO correctly.
While a higher ISO is good when aiming for a quick shutter speed, it also introduces a significant amount of grain or digital noise into the image. That’s something you want to avoid, and it’s the reason that you’ll often hear that you should always use ISO100.
To achieve the longest possible exposure I could use an ISO of 64 here.
Now, I agree that you should aim to use ISO100 for most stationary landscapes, you shouldn’t make the mistake of only using that setting. It took me several years before I managed to accept that there’s not only one correct ISO in landscape photography. In fact, I was pretty much an ISO100-nazi, and except for night photography, I stuck to it.