Airline run flight schools in Indonesia undergo quite an involved selection process involving psychometric testing, skills test and interview. Even during the pre-PPL stage, some schools monitor the progress of their students and those who don't show any progression are dropped out of the program. The same goes for the student pilots at the national flight school STPI.
I think the 80% failure rate is anything but a guess and a very bad one at that. The blog author was only in Indonesia for a very short time I believe.