You cannot convert a newer file format into an older one.
What you have found is a description of exporting the data from the newer format file to a file type that can then be used to import the data from the export file into the older format file.
This only transfers the data. Calculations, scripts, layout designs are not part of this transfer. You'd have to manually recreate those features in the older format file.
The data transfer would look like this:
In the .Fmp12 file, show all records and then use export records. A useful file format is often to export to a file merge file as this includes the field names as a header line and can be used during import to align fields from the file of exported data into the older format database file. Then open the older format database and use import records to import them.
You'll need to repeat this for each table that has data you want to transfer to the other file.
This process can be scripted.