They propably are just newly hatched ones that came out of brassica leaves you've put into heap. They won't be able re-produce in there anymore, but will need live plants for next generation. So what you've seen now propably are just about last lot you get unless you compost some more fresh leaves.
There is no worries about spreading the problem..what you've got out of bin into air is about as bad as adding a drop of water into sea and having a fear of causing flood... ::)
White fly is all around us..by their millions and there is no way of ever getting rid of them. We might just get rid of few from odd plant but without repeated sprays..they just keep coming back.
So..carry on composting and don't worry.. ;)