I'd suggest you use what suits your needs, and your experience. If you are experienced with Dusk and it suits the requirements, then by all means use that, why be forced to learn Selenium if it's not needed. Sounds like the CTO is opinionated and likely has prior experience with Selenium.
The important question to ask:
- is he working on the code base or building any tests?
- what are his reasons for using Selenium instead of Laravel Dusk?
The difference between Dusk and Seleium are:
Dusk:
- PHP
- Browser Automation
- Used for browser automation and testing API
- Does not support Fixtures, Group fixtures, Generators unlike Selenium
Selenium:
- Python
- Web Automation
- Used for testing web applications
- Supports everything Dusk dues in addition to Fixtures, Group fixtures, Generators.
Other than that they are pretty much the same. Depends on what you are testing and your requirements really.