Ah, troubled by a colleague that has not yet agreed with the choice for Mendix and who is afraid of his expertise becoming obsolete.
For your argument: Sorry, but yes, it is possible to insert data directly into the database if you create/copy a SQL-module. There is no SQL-module in the Marketplace. So you cannot say that it is not possible, but it does require a lot effort to set it up.
Even if you have set up an SQL-editor, when inserting the data, you will have to assign ids to your objects. So you will have to find out how Mendix does that, and do exactly the same. One failure immediately corrupts your database.
And you will have to prevent other users interfering with your commits.
Also: why would you not just use Mendix? It has great tools for importing data.