Standard Enthalpy of Formation

I have hard time understanding why in the definition of standard enthalpy of formation (SEF), the reactants and the products are required to have fixed temperature and pressure. Specifically, LibreTexts define SEF as follows: "SEF is defined as the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298K) is formed from its pure elements under the same conditions" ([link here](https://chem.libretexts.org/Courses/Valley_City_State_University/Chem_121/Chapter_7%3A_Thermochemistry/7.4%3A_Standard_Enthalpy_of_Formation)).


My question is the following: consider the case where we want to get 1 mole of O, given 1/2 mole of O\_2. So, we start with a container with 1/2 moles of $O\_2$ in 1atm, 298K. Then, we add heat to break the covalent bonds. Suppose we managed to have only isolated O atoms in our gas. However, the temperature and possibly the pressure are higher than the desired values (1atm, 298K). By adjusting the volume and the temperature, we can then reach the desired values. So now, ideally, we have 1 mole of isolated O atoms in 1atm, 298K. However, this is an unstable configuration, so we will not achieve it. Am I missing something here?


