For an ideal gas, internal energy and enthalpy depend solely on temperature; therefore, when temperature is constant (which is implied in standard thermodynamic contexts when both pressure and volume remain constant for a fixed mass), their changes are both zero and thus equal, making this a matter of fundamental thermodynamic definition.
The claim relies on the standard definitions of ideal gas internal energy and enthalpy, which are functions of temperature alone. When pressure and volume are both held constant, the system's temperature cannot change for a fixed amount of an ideal gas (pursuant to the ideal gas law PV = nRT), meaning the temperature change is zero, leading to zero changes in both internal energy and enthalpy. This is a basic matter of textbook thermodynamic definitions (common knowledge) rather than a contested empirical hypothesis.