Every contract has an out for non-performance. Horizon, for whatever reasons, has manifestly been failing to perform. AAG is quite unhappy that Horizon is dragging down the brand. "Brand" is of the utmost importance to Alaska, it is all that they have to fend off Delta. Deferring deliveries is an obvious, and undoubtedly necessary, remedy in the short term. This will give you some time to fix staffing and performance problems. The reason they are not forthcoming with specifics is undoubtedly because they are dependent upon Horizon resolving it's issues.
When Horizon has adequate staffing and acceptable performance, more aircraft will be forthcoming. In six months, if the problems aren't fixed, expect more deferrals. Eventually, AAG will pull the plug.
You've got maybe a year.
Horizon's pilots are in no way at fault for this situation, but beligerant actions by the union are likely counter productive.