Andrew, the Lead Owner field is a system field and cannot be updated with a list upload. That is because the field always contains the SFDC Lead Owner (and only for Leads that are synced with SFDC). The only way to change lead owner is via the Flow Action "Change Owner".
You could make a custom field to upload the lead owner into, then create a Smart Campaign that will trigger on a data value change and then use the "change owner" flow step to change the owner in SFDC. BTW: if the lead is not yet in SFDC, it will be synced to SFDC when you use "change owner".