The microflow commit is an action where the object is persisted into the Database. Until the user clicks save button, the employee object need not to be saved in Database. The microflow just before this action was to create a fresh employee object. That's why there is no retrieve associated with the employee. Where as an account is always retrieved from the database and a retrieve is needed.
The best practice is to reduce the number of commits in a microflow as the commit involves a DB write operation.
Actually, I finally made a simple experiment. I removed the Account retrieval. After that, Account was simply not available and could not be selected for commit.
When Account was retrieved, it could be commited afterwards. So I think this clearly shows that when the input parameter is Employee object, we have no direct access to Account object. But we can have the access if we do Account retrieval using associations (=ORM relations) and after that we can do the Account commit.