Hello,
phpxlsx is available in phpdocx Bureau licenses or as a standalone library (https://www.phpxlsx.com/). phpxlsx is not included in phpdocx Premium as you can read of the features page (https://www.phpdocx.com/features) and the pricing page (https://www.phpdocx.com/pricing).
If you want to use phpxlsx you need to upgrade to phpdocx Bureau or buy it as standalone library.
phpdocx Premium includes some specific methods to work with XLSX and PPTX files you can read on https://www.phpdocx.com/documentation/api-documentation. But it's recommended to use phpxlsx to work with XLSX files and phppptx to work with PPTX files, that are full libraries to work with these file types.
Regards.