Marketo currently does not automatically sync leads to SFDC.
If a lead is synced to SFDC, this means that you have something configured in your instance to do so, whether it's a flow action of implied sync through Marketo program>sfdc campaign sync or some other process.
So many systems were setup to sync directly to SFDC based on MQL rules or Program-SFDC Campaign sync. Robin Root you should see if these rules would send your list to SFDC, or if it would only send some.
Like Jim suggests, you could use Member of List to the Sync Exclusions.