If these lists are imported into Marketo, the people will not automatically be synced to SFDC. If they are part of a program synced with SFDC or if they run through a "Sync person to SFDC" flow step then they will be synced. I would look into the activity history of one of these people and try to determine where and how they were synced into SFDC. Then you can adjust the sync to only sync those that have not hard bounced.
That's what I thought, but most of marketo programs are not tied into SFDC campaigns at the program level. I've identified a few smart campaigns based on the lead's activity history, but the campaigns don't have "Sync person to SFDC" flow step. I wonder what's causing them to get synced to SFDC...