Hello,
As CodeIgniter 3 doesn't use PHP namespaces, we recommend you to use the classic package (although the namespaces package could be use as well, and it's the recommended package using CodeIgniter 4) and add phpdocx as any other external library:
https://stackoverflow.com/questions/45009712/load-external-library-in-codeigniter
https://forum.codeigniter.com/thread-49644.html
The main CreateDocx.inc class loads the internal autoloader to use the library. You shouldn't find any issue to use phpdocx with CodeIgniter (or any other framework, CMS or custom project).
On https://www.phpdocx.com/documentation/cookbook/integrate-phpdocx-codeigniter you can read the documentation about using phpdocx with CodeIgniter.
Regards.