No the user does not know who it was, it happended pogacar his activity got flagged. segments that have been flagged as dangerous for some sort of reason are also annonimous, the creator of the segment doesn't know and the segment gets hidden on activities, I have seen over the years someone flag segments in certain areas and there was nothing dangerous.

Are you referring to flagging an activity or flagging an entire segment?  Either way, other people do not see who it was who flagged it.  Entire segments should only be flagged if they are clearly dangerous to others.