The phrase "The highest as the lowest form of criticism" means that the highest and lowest forms of criticism (and by implication all the other shades of form between them) share a characteristic: in this case being a mode of autobiography.
It is similar to saying "The highest and lowest forms of criticism are both modes of autobiography" or "...are similar in that they are modes of autobiography". The closer of your two interpretations is "The highest as well as the lowest" but this does not quite encapsulate the idea that both forms of criticism share being a mode of autobiograhy as a fundamental characteristic.
Having said that "A as B are" is certainly not a common idiom. It sounds both stilted and dated and probably sounded a little unusual, if not downright odd, to Wilde's original Victorian readership. Wit though he was he took literature, himself, and his reputation very seriously and is known as The father of Aesthetics because of his involvement in, and development and promotion of, the Aesthetic movement so the use of an unusual turn of phrase is consistent with his general approach to life