As per the article.
New joiners in 2017 = 634 ( new Fo's) mainly
Leavers 2017 = 718 (experienced)
Net loss = 80pilots over all but 700 experienced pilots gone.
New frames 2016-2017 = 100
10 pilots per frame = 1000 extra pilots needed to fly those frames..
Add in the 700 experienced guys that have left.. you could say they are 1700 experienced pilots short to run the fleet.
Obviously it can be interpreted differently but outgoing experience can not be balanced with inexperienced new pilots.