Currently alwaysUse24HourFormat's documentation only discusses what happens on iOS and Android. This should also include desktop platforms (Windows, macOS, Linux) as well as whatever happens on Web. Looking through the engine code for the web, it seems like it's always set to false unless I'm missing something.

For reference, here is the current documentation:

Whether to use 24-hour format when formatting time.

The behavior of this flag is different across platforms:

• On Android this flag is reported directly from the user settings called "Use 24-hour format". It applies to any locale used by the application, whether it is the system-wide locale, or the custom locale set by the application.
• On iOS this flag is set to true when the user setting called "24-Hour Time" is set or the system-wide locale's default uses 24-hour formatting.