When a Lead or Contact is deleted from Salesforce, Marketo still maintains the SFDC ID and SFDC Type, even though that record has been deleted from Salesforce. I think we need a better way to filter on Leads who currently exist in Salesforce.
It's possible that someone could be deleted from Salesforce, "SFDC is Deleted" would be set to true, and then that Lead syncs back to Salesforce.... "SFDC is Deleted" would still be true in this case.
A filter for "Currently Exists in SFDC" would be ideal to get around these nuances.