On the other hand, he just assumes that two straight lines can only intersect once, and that this describes space,
I don't see this in the quoted passage. He's trying to illustrate the nature of propositions in geometry, and doesn't appear to be arguing that the parallel postulate is universally true. "Take, for example," is not exactly assertive.
Also, have a care: those two paragraphs are not consecutive in the Critique.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
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