The website doesn't even mention FFmpeg which is the open source tool that does all the heavy work. Building a user-friendly GUI is a valuable work in itself (congrats on that, sincerely), but it's not okay to hide the open source tool that does all the work in the background.
It should at least be visible in the FAQ that the open source FFmpeg tool is used by the app, with an appropriate link to FFmpeg's website. The tool could be described from the start as a "User-friendly FFmpeg front-end" (but I would understand that this may not speak to its target audience, hence the idea of putting the information way down the page in the FAQ or even the footer, but at least don't hide it).
In general I agree, it gives me a sense of wholesomeness when companies open up about this and give credit to the authors of the libraries that their product contains. Credits.md somewhere is sufficient for that to me.
It should at least be visible in the FAQ that the open source FFmpeg tool is used by the app, with an appropriate link to FFmpeg's website. The tool could be described from the start as a "User-friendly FFmpeg front-end" (but I would understand that this may not speak to its target audience, hence the idea of putting the information way down the page in the FAQ or even the footer, but at least don't hide it).