One Shi’a Scholar said it’s not right to use the term wajibul wujood or ‘necessary existent’ as the Ahlulbayt as never used this term. So is that right?

The criterion about what can be used or not used is whether if not it contradict the sharia or not. Yes something may have not been used or said bu the imams that however does not make its use deemed unlawful. E.g. loud speakers were not used by Imams as they did not exist , mihrab one the mosques were never known during their times, minarets etc etc