B.S. plain and simple. They want your money. I'll promise you, there's no way anyone is going to upgrade to captain in 3-6 months. Kind of strange they can't name the "airline" they guarantee this with.
Bottom line is that in this industry, most of the time you're gonna have to put in the time flight instructing and flying odd jobs to build up time/experience. A "guarantee" like this is akin to someone promising a first year medical student a chief of staff position at some hospital in one year after paying X number of dollars. Just doesn't happen.