Uexkull's work, and formulation of the Umwelt must be contextualized in his predecessor Kant's ontological framing to be understood.
Based on the Newtonian mechanistic view of the world, Kant postulated that there must be some knowledge that must be known about the world prior to a (human) being being born into the world and called this synthetic apriori knowledge.....namely time, space and causality, the things that a Newtonian, mechanistic worldview assumes at the outset.
Since any object of the world can only be known through the 5 senses, we are estranged from reality, and there must be some knowledge we must have prior to sensing the world that helps us make sense of it.