Well, I see what you are saying. So I wonder....
Alex, "what is out there" after remove human or alien, is classic idea of "objective reality".
But note: since reality is ALL that exists; then as humans (and maybe aliens) exist, and as their thoughts, opinions, hopes, fears, and relations with "what is out there" also exist; don't forget all those phenomena are a part of reality too.
Even a fantasy has this much reality: it is a real fantasy (eg. see the movie: "Monster's Inc."; (a good introduction to parallel universes)).
Dick does not seem concerned whether his "numbers" are "events", "actions", "objects", "patterns", "relations", "comparisons"; he seems only concerned that they are distinct*** from each other such as to be able to be counted.
So he calls them :"a set of numbers"; I prefer "a set of distinctions"***.
True, he wants to avoid making assumptions about reality, alien or otherwise. So he takes the apparantly super-basic concept of "countability"; and looks at the consequences of various ways of shuffling that which is countable, doesn't he?
True, he uses human inference when extending his observables to science- I think there is another logical assumption that he makes. He builds his castle on a foundation of mathematics; without adequately analysing what it means for something to be "distinct", to be "countable", to be "re-arrangeable" or "mappable".
I think a more accurate approach is to start with "existence"; figure out what has to occur for the detection of existence of a phenomenon.
Figure out the structures beneath mathematics.
Curiously, in chapter 5 he apparantly claims that only events have dimensions; that there may be zillions of dimensions; and this suggests to me that his initial "3-D" space + 4th-D for change; is actually something more basic than "dimension" or "space"; but what I call a 3,4 interchange system that seems to be related to the idea of "objects" as being a 3-in-1 mimimum network of agreements to give stable object.