Where do see 'Sync Implied'? Do you see in a system managed field, sync activity, anywhere else? This probably relates with the implied salesforce sync actions, where-in when the flow step/activity that imply other primitive/implied sync actions is ran, the implied actions/activities are performed automatically. For example, when a Marketo only record is passed through the Change Status in SFDC Campaign flow step, then the record is inserted in SFDC, and is added to the respective SFDC campaign first (as these are implied actions/activities that need to happen before changing the campaign status).
Well, the sync status of the Lead, Contact, or Account object is changed to Sync Implied when one or more custom objects linked with the respective standard objects (Lead/Contact/Account) are enabled for syncing with the Marketo. I believe, it essentially means that the custom object data linked with the salesforce object (Lead/Contact/Account) is syncing with the Marketo. Also, when a high level CO is synced, Marketo syncs the associated/linked dependent fields on the custom entities below as well (i.e., their sync is implied). As you would know, when none of the linked COs are enabled for syncing with Marketo, respective standard object's sync status is set as "Sync Disabled".