Yes they can make bookings now with their seniority.
A few people that started their careers with British Caledonian and Dan Air etc. have incorrect start dates. ie. when their previous airline became part of BA; and these more complicated start dates are being entered by hand and will take a bit longer to sort out.
Basically all the people who started with BA have the correct staff travel date and others who started their career with another airline can use their concessions but their correct start date will be entered as soon as possible and the relevant seniority will then be corrected.