The QStyleHints documentation says:

Detailed Description

An object of this class, obtained from QGuiApplication, provides access to certain global user interface parameters of the current platform.

Access is read only; typically the platform itself provides the user a way to tune these parameters.

Access to these parameters are useful when implementing custom user interface components, in that they allow the components to exhibit the same behaviour and feel as other components.

The red highlighted text is misleading since there are a number of setter functions.