Many online converters process files on remote servers rather than locally in the browser. The file lives on someone else's machine for the duration of the job, sits in their object storage while their retention policy ticks down, and shows up in their access logs. For casual home video that is usually fine. For internal work clips, footage you would rather not hand to a third party, or anything you would prefer to keep on your own machine, sending it to a stranger's server is something to consider.
This tool was built so the model is structurally simpler: there is no server-side upload endpoint for video at all. The site never receives the file in the first place. Conversion runs in your browser via the WebCodecs API, on your own device.
You can verify this in about 10 seconds: open your browser DevTools network tab, run a conversion, and observe that no outbound request carries the file. The page itself is just static HTML, CSS, and JavaScript.