I don’t think that I can agree with Kant on that, although perhaps it is an issue of clarification.
Is it an example of “Ding an sich” that what is on the table is “an object”?
If so, then that can be certainly known, although not as trivially as most people would think. The fact that you see what you think is an object is insufficient proof. But cognition is merely a matter of category definitions. An object is defined to have specific characteristics. And those characteristics are related to affects upon the individual. The characteristics and affects can be defined and directly known if defined properly.
Only an object being present can have the affect AX on me
The affect AX is upon me
Therefore an object is present.
Defining AX exactly right is the only issue. One cannot say that only an object can appear to be an object. AX cannot be simply the perception of an object. AX must exclude any possibility of anything other than an object.
The mind can only know perceptions, definitions, and conclusions. When the definitions are properly related to the perceptions, certainty of perceived experiences (knowledge of “ding an sich”) can be obtained. An example might be:
Def: Reality ≡ the only cause of perceived affects upon me.
Ontological principle: All perceived affects are caused by reality.
Known situation: I perceive affects upon me.
Known conclusion: Reality is affecting me.
Note that this does not exclude the reality of physiological or psychological conditions bringing about aberrations of unrealistic beliefs that can mislead to false knowledge. It is still reality that is affecting even though it is perhaps a fantasy that is believed to be that reality due to presumptuous processing or reasoning in the mind.
The concern is ensuring that the definitions are exclusive; “By definition, only X or Y can cause Z”. There must be a complete lack of alternatives to ensure certainty of conclusions. Thus our “affect AX” must be defined as only possibly caused by an object being present. One must be able to accurately say, “I know, without room for question, that an object is present simply because I perceive affect AX.”
Unfortunately, lazy egocentric philosophers want to take the easy route of defining the perception itself as the only reality (relativism) or that the fantasy within the mind is the only reality (solipsism). Objective reality is known for certain only through cross verification, known as “falsification”.