إذ الواجب لا يكون إلا قديما
For the Necessary Existent can only be beginningless.
Here qadīm denotes what has no beginning to its existence. If the being necessary in itself were supposed to have been nonexistent and then to have come into existence, a further explanation of its origination would be required. That would conflict with the meaning of necessary existence in itself.
Thus necessity entails beginninglessness, but the terms differ conceptually. Wājib concerns existence belonging necessarily to a being itself; qadīm concerns the absence of an initial point. A necessary implication is not the same as complete synonymy.