Accelerate never collects any personal data or identifiable information from you.
Safari shows this general warning for extensions that load JavaScript into all web pages, which includes most Safari extensions on the App Store. In the case of Accelerate, the extension runs code that finds videos on each page you visit, and adds your playback options to control them. Apple rightfully cautions the user in case a malicious extension tries to abuse this permission; however, Accelerate never attempts to read any sensitive info from a webpage, and does not need to collect any data to function. If you're interested in learning more about privacy with Safari extensions, I recommend checking out this excellent blog post.
If you have any additional questions about Accelerate and its privacy policy, feel free to contact the developer via email.