So, you're looking for Leads that are Marketo-only, regardless if they were ever in SFDC or not?
If so, I think two filters would be needed:
SFDC Type = Empty (so you get all your Marketo-only names, who were never in SFDC)
SFDC is Deleted = true (Marketo-only names that have a SFDC type, but have since been deleted)
That should show you everyone who is Marketo-only...in my mind at least!