Someone from B6 will have to answer your specific question, but in general airlines are *not* required to have completed background checks or SIDA checks before you start class... obviously that will need to be done prior to IOE (sim?) or granting JS access, but some airlines will let folks start class with the backgriound check pending. It's usually the regionals or budget airlines... they know they can always fire you if the background comes back with any issues.

The top tier generally will not give you a class date until the checks are done, they don't want to deal with awkwardness of the applicant having quite their previous job and possibly having given up years of seniority.