You have hit the nail on the head, in RYR it is possible (and it would seem quite common) for someone to move from F/O (senior) to Line Training Capt within 2 years.
I then have the "pleasure" of them on the jumpseat conducting my annual line check, giving me (a 15,000 hour Capt) the "benefit" of
their vast experience !
RYR are just looking for people to be SOP "police" and to train another generation of SOP robots IMHO, and there is no shortage of young eager twenty something year old Capt's on an ego trip willing to do it .........