At the moment I'd guess might also be to do with the cut down nature of the preview. Even the full rulebook is unfinalised.
Since characters can definitely come in from pure technical streams, I'm not sure the test pilot thing does apply. One character could be a totally civilian engineer or technician with the same skill set as a Military Support one. Both characters could get into space through the same commercial or mission specialist paths, despite the military one never having been commisioned.
Having said all that... Rank 0 could be any noncommissioned rank, I guess. And the event table has the standard Traveller automatic commission or promotion text, despite there not being any other mention of commissions in the preview. So I do suspect the full rules are more like standard Traveller.