ike,
I'd guess you're right about the small(ish) number of Aussie applicants but what makes this story unusual, if accurate, is that any US airline actively sought expats. While I may have overlooked some event in history, I don't recall this happening before. Someone will no doubt correct my memory if needed.
The ad from the Skywest website seems to indicate a CPL is good enough to get the ball rolling if the applicant has the necessary flight time. Am I reading that right ? :
Pilot » SkyWest Airlines
It's more likely that younger expat applicants might have an FAA CPL than an ATP.
As for seniority, wouldn't the Aussies just drop off the list when their visas expired/weren't renewed ( to the pleasure of those junior to them) and go elsewhere ?
I guess this gets complicated.