It's probably useful to understand exactly what's happening in this situation. What's happening is that the low-res thumbnail images were created/modified by an older app that generated a low-resolution finder icon and applied it to the image.

If an image has NO custom icon, the Finder will generate one on the fly, which will look good at high resolutions (of course). If the image was saved by an app that creates a high-resolution custom icon (or a high-res preview image), then the Finder will use (in most cases) that image (this also applies to aliases, as described by another commenter). But if it has an old, low-res custom icon (generated by a Classic app, most likely), the Finder will use that, and it looks bad.

The hint is just describing how to batch-remove all custom icons from a bunch of image files, which leaves the Finder to generate new ones on the fly. The problem is that in some situations (such as aliases), the Finder won't bother to generate an on-the-fly preview, so you end up with a generic image icon. I, personally, prefer this behavior; since I mostly do web work, I like my images with the least embedded metadata possible (which is to say no custom icon, etc).

But others with different workflows may have serious issues with this (for example, if you rely on aliases, which won't get previews generated on the fly, but do inherit custom icons); what they need to do is find a method to batch-update the custom icons, rather than just removing them. I don't personally know of any app that will do this, but wouldn't be surprised if one exists.