USB-UART FTDI chip VIH
Your interpretation of the specification is correct. The input threshold is set by an internal reference that is nominally 1.2 volts, regardless of the operating voltage.

The point is to be able to interface with a wide range of logic voltage levels (like your 2.4 volts) while still powering the device from 5v or 3.3v, without having to provide a pin for you to separately provide the logic interface voltage.