I’m currently on the same issue. I also tried with a “standard” mt file, e.g. from here: https://www.sixapart.com/movabletype/docs/mtimport
But I don’t get the posts. I get the categories, the authors and the comments.
When I checked the database I see in the wp_posts table all post entries, but the category ID of all posts is “0”. If you change manually this entry to the correct category it will work.
It looks for me to a bug in the import php-file, but I’m too far away from php to figure it out.