ATS requires a Selenium environment as you can find on the following page: https://docs.mendix.com/ATS/general/deployment.
The Selenium Runner is used to execute your tests. Once you trigger a test run from ATS, ATS will connect to your Selenium Service provider to start the test. The Selenium Runner then starts the browser and executes the commands that it receives from ATS.
Please find the compatibility list at the following URL: https://docs.mendix.com/ATS/general/compatibility.
I have tested ATS with some projects and it works very easy.
I'm wondering where you heard that ATS requires a third party tool.