Not sure what GP meant, but I'd guess promotions are much more susceptible to discrimination cases than hires, since there's something much more concrete to compare against.
If you hire only one gender with narrowly targetted job advertisements, the"wronged party is a pool of thousands (or more) of engineers that didn't even know the job existed. If you promote only one gender from the mail room, the wronged party is a small cohort of peers that can visibly see exactly who is promoted above them. Those are much more dangerous waters to tread.
Australia BTW.