Thin fonts are “thin” in the sense that they do not have much line width. In font families, this style is often called Light fonts, Extra Light fonts, or Thin fonts. A font can be thin or light without necessarily being condensed.
We could consider this font both thin and condensed at the same time. Its proportions are condensed, and the lines themselves are thin. In fact, thin fonts can make for a really elegant design choice. Note how the font lends itself to an aesthetic that feels serious. Think about what you would assume about this book, based on the cover alone. The font choice really has an impact on how this visually communicates.