Hi Glowe,
as Jan already said, using the featured image as thumbnail is a common practice in many professional WordPress themes. You can use a plugin to add the featured image to the old posts, if you prefer that instead of adding this manually.
Or you can edit the theme files and remove the image / place holder completely, if you prefer that instead.
About the FAQ, I said that you can find more instructions there regarding the theme setup. I never said that you can find an solution for this specific thing. I just wanted to help you with the theme setup, sorry.
Btw. this theme is used on thousands of websites and actually people are very happy with it. So I think it is worth to call it a “real” theme. But if you don’t like it, you don’t have to use this FREE theme and try to find another theme instead which fits your needs. ??
Regards
Michael