Being different is only allowed today as a chosen, not exclusive, not fated, but constructed and constructible being different, that at any time therefore ought to be able to be discarded, replaced or assumed like a pair of glasses, a beard, a fashionable opinion. Whereas every inescapable being different must - as here intrudes the final involvement of the Weltinnenraum - be deconstructed, broken up or, at least, relativised in its seriousness.
What does this mean? It's the basic point I make so often at this site. Liberal modernity holds, as its overriding good, a freedom to self-determine or self-define. Therefore we are only allowed to have a self-constructed, fluid, changeable, discardable identity rather than one which is predetermined or fated. The inescapable forms of identity are held to be oppressive and earmarked for deconstruction (or, as Kubitschek points out, are made less serious by no longer being thought to represent something objectively or inherently good, but only of value from a subjective, relative point of view). Some of the inescapable forms of identity include our membership of an ethny and our biological sex.
Kubitschek asserts that a reason why liberal modernity rejects predetermined differences between people is that these differences bring us to an awareness of the "Weltinnenraum" - a concept popularised by the poet Rilke. The word means literally "world inner space" though I have seen it translated as "outer space within." It seems to refer to the moment that we perceive the interpenetration of the outer world with our own inner one, or an awareness of a transcendent nature through which we have our being. Liberal moderns, according to Kubitschek, are discomfited by this engagement with a larger, non-material aspect of their being and therefore turn from it, to a smaller understanding of the self as being self-constructed.
I'm not sure about the Weltinnenraum aspect of the quote, though it's an interesting thought, but the quote in general is eloquently expressed.