That's an unwarranted assumption. In almost all cases, not-logged-in or non-elite availability at the lowest level in an airline's native program does mean that those same seats are available to their alliance partners.

For example, every time I see that aa.com has low level availability, that same flight is available for Alaska Airlines Mileage Plan. And they're not even in oneworld. So unless LAN is engaging in the oneworld equivalent of the infamous old-United "StarNet Blocking" (a procedure where UA's computers were programmed to hide *A-wide availability from partners on flights UA considered too expensive), then this is IT incompetence on LAN's part. Not AA holding back the availability.

I'm pretty sure that AA does not "release those seats to LAN" but rather AA posts those seats to the alliance (and nonalliance partners) as available. It's not "3 seats to Alaska Airlines, 5 to BA, 2 to QANTAS, nothing to LAN". It's "5 seats to whomever wants them to any of our partners if our own members don't book them first". The same 5 seats.