Such a trifling thing is not worth keeping. Let it go along with all the rest' -then, then it is acosmism. Indeed, if the intention were that he should give up the whole world and be satisfied with another's person's ethical actuality, well, then he would be right to make light of the exchange. If it is not so, it is the worst for the individual himself, because then he has nothing at all, no actuality at all, because to everything else he has at the very most only a relation of possibility. Here are some thoughts of mine to take the potshot at.
Ethical actuality is to stand in that space between thought and being where thought becomes action.
The conditioned self acts as a bridge that connects the two and allows no space between. The breaking of the conditioned self is the subject of Part I of K's book Judge for Yourself . Read it, then judge for yourself if that is not what he is speaking to. I'll add more when the spirit moves. Your analogy with solipsism was a stroke of good luck.