Ok, this is a good criticism. Particularly your last sentence. I shall try to address it.
In a sense you are completely right that it is idealism of the highest order as it specifically excludes one particular question from study - taking it as inaccessible to study, in fact. This is my position, pretty much: The fact of existence is not something we can say anything meaningful about - it is simply something we have to accept. But it is the only thing we cannot say anything meaningful about. It is the nature of all complete systems that they cannot completely prove themselves from reference only to themselves. If you want to call that idealism, you are not entirely unjustified. Nonetheless, I think it is provably true.
As for 'knowledge can only arise from acting in the world'. Yes, but I think this is a circular statement. Knowledge and the world coexist. There cannot be one without the other. This is why I said that I consider 'existence is' and 'knowledge is' to be effectively equivalent statements.