Thanks for a reasonable question.
In the taxonomy menus, the Name, Description, Slug and Count columns are sortable because they refer to columns in the wp_term_taxonomy database table. The “Attachments” column is not stored in the database; MLA calculates it dynamically when the Media/Edit Taxonomy page is accessed.
Because it is not stored in the database there is no way I can think of to sort the submenu table on the column.
I regret the news is not better, but I am marking this topic resolved simply because the question has been answered. Thanks for your understanding and your interest in the plugin.