I don't see how the court can rule that a state isn't impacted by the illegal behavior of another state in the process to elect a federal government that preside over all states.

If a state decided to assign its electoral votes according to the results of gladiatorial combat, the rest of the states would be well within their rights to either reject those electoral votes or withdraw from the compact altogether. Clearly, there is a range of acceptable processes for valid contribution to the federal election, and that range is defined by local election law. Texas sued precisely those states that broke their own laws, correctly reasoning that Texas should not be bound by a compact with parties who do not adhere to the norms established by their own laws. Put another way: why honor a contract with states that are actively breaking their own contracts?