To be noted: They were Expats working in large companies, and the person they dated didn't work in the same department than them.
Girls go to clubs to meet guys, they get drunk, they flirt, etc...
For tricks about meeting girls in Jakarta bars & night clubs, I wrote a special post available here: Which bars to find girls in Jakarta?
and you can also read my list of Ladies' nights in Jakarta. Where do you think are the good areas to find "normal" girls in Jakarta?
Your objective should only be to get a phone number, nothing more.
Some places are more easy within the mall of course, like the cinema, the gym, or any queues... You have to be very careful, but if you are discreet, it can work well and it made at least a couple expat friends happy.
It may be a birthday party in Dragonfly, a wedding, a dinner between colleagues.