The import script was written to import to a version prior to UBB.threads 7.7.x
From the importer's header:
* UBB.classic Import Script Version 4.0 (May 27, 2006)
* for UBB.threads 7.0 and Higher only
I know it says "7.0 and Higher only." But I am confident the author of it would not have been able to predict PHP 7 at that time, with a requirement for MySQLi.
The importer script requires MySQL and PHP 5. Which would be any version of UBB.threads 7.0.x - 7.6.x