I read today in the short Vedic text Laghu Vakya Vritti that the teacher of Vedanta is a man of Self-knowledge and also possesses deep learning of the scriptures. This is a very important point. Self-knowledge (ie enlightenment) is a separate issue to knowledge of the scriptures. Said in other words, fullness of experience is not necessarily fullness of knowledge.
It seems to be a very common experience that people get enlightened and don’t know how they got there. As such they are of limited help to seekers. In the same way, a lottery winner is not necessarily an expert in money. Ideally, you want to take financial advice from someone who 1) knows about economics, finance etc 2) has turned that knowledge into money. Likewise, a teacher of Vedanta needs both knowledge and experience to be properly qualified.
Also, if a teacher has an education in Vedantic scripture, they are more likely to belong to a tradition and have had the benefits that only come from a guru-disciple relationship. Unfortunately, such teachers are extremely rare.