"allow creation...." enables you to enter data into the blank "add row" of a portal to the related table and then FileMaker will create the related record and copy over the needed match field values. It doesn't cause your table to automatically create a related record for every record in another table. It can also make it possible for Set Field--if performed from a layout based on the remote table, to create a single related record in the new table.
If you want one related record in this new table for every record in your externally sourced table, then you'll need to set up a script to loop through the records of the external table and create one new related record in the new table.
So is my statement:true or will the new field be protected by the fact that it doesn't exist in the EDS?
None of what I describe requires creating new fields in the external table as long as you already have a primary key field in the external table that you can use to define a one to one relationship to your new related table.