The developer did not prevent you from creating new layouts. Each new layout must have corresponding code in your theme to display that layout.
Somewhere in the code of the theme there will be a call to the function get_row_layout(). This call returns the layout name of the current layout in the flex field. This code will be followed by code created specifically for displaying the fields based on the layout name returned.
Different developers do this in different ways, but without adding the code for the layout the layout will just be ignored because the code does not know what to do with it.
