Knowing God and comprehensively grasping the divine essence are different claims. Certainty about existence, unity and attributes does not require a complete classification of the essence in a genus, differentia or material structure.
In many areas, knowing an established fact differs from exhausting every detail about its subject. Here that serves only as an analogy for the distinction between knowledge and exhaustive comprehension—not as an analogy between God and a created entity.
The important caution is not to confuse lack of exhaustive comprehension with lack of knowledge, or to fill an unknown modality with invented bodily descriptions.