I got this question by email and it's a good one because the book (Quantum Void) never explains! Slight spoiler alert.
First, Aastazin (Zin to his friends) couldn’t have arrived on Earth via the portal because he had to first get to Earth to build it. So, he must have arrived via the black wedge that was hanging around the lost Soyuz capsule and was later spotted orbiting Core (Quantum Space). Perhaps the wedge has a passenger compartment inside and it got very close to Earth from a 4-D perspective, so close that Zin simply stepped out on the surface. Since Zin isn’t biological, the transition from 4-D to 3-D didn’t kill him. Maybe he just materialized in NASA’s lobby at Kennedy Space Center and caused the poor receptionist to freak out? Just a theory. 😊