Indeed.
I didn't do a tax return last year either, it is due annually in June I believe, so I was a few months overdue.
I paid 25k to my accountant to sort it all out, and had an outstanding tax bill of 60k, which has now turned into a rebate of 120k, so I am OK with that! As you state there was a 400thb fine for filing late, paid to the Amphur.
Those bringing money into the country thinking that they can just keep quiet and that they won't be affected by this may be unpleasantly surprised.