Upon viewing more than 30 seconds of the video, it does appear to only deal with UI edits. The emphasis on "No coding required!![sic]" didn't make that obvious at a glance.

Yes, I agree with you there, the page does not do a very good job of explaining exactly what is being demonstrated. If you start on the site's landing page [0], it makes it a lot clearer.

[0]: https://www.vessel.io/

Co-Founder of Vessel here, thanks guys for your feedback. We will reward it as, A/B testing without any code changes.