When you use the "Add to SFDC Campaign" flow step, and the lead isn't already in SFDC, this step will sync the lead to SFDC, and you want to know which assignment rules will be applied.
It's not just "Add to Salesforce Campaign" that uses what is called "implied Salesforce actions". All of the following flow steps will sync the lead to Salesforce first before running the flow step if the person is not already synced to Salesforce: