Answer
The statement is consistent.
Work Step by Step
The statement about so-called “state variables” applies to a substance in one state, such as a liquid or a gas. When the substance is undergoing a first-order phase transition, as when a liquid evaporates to a gas, it is between 2 states and the system volume can change without changing the temperature or pressure.
In other words, there is no reason to expect one equation of state to hold for a system containing material in 2 states.
