To cull something means 1 reduce the numbers of (animals) by selective slaughter. 2 select or obtain from a large quantity or a variety of sources.
It looks as though the second meaning is the one that is being used - He is deriving information from the conversation. Not really a particularly good example of usage - the sentence looks unfinished.
I agree. It does look like the subject of the sentence has selectively taken information from the conversation...though it does seem to be an incomplete sentence since the immediate thought that comes to mind is...What did he cull?
If the subject was somehow removed from a group of people having a conversation, the sentence might go
" He was also culled out from the conversation along with others who disagreed with the majority opinion."