I haven't checked their specific app ($119.99) but it's common for packages to have OSS attribution and copyright notices as a dialog or something that the user can click on to see. Since they're presumably not modifying ffmpeg, there's also no source that needs to be provided.
For example, my Chrysler car infotainment has an option in the system settings to see all of the copyright notices and OSS info that goes into the system.
For example, my Chrysler car infotainment has an option in the system settings to see all of the copyright notices and OSS info that goes into the system.