The error message doesn't provide any sort of insight into whether or not that's the case. Contrary to that, the upload box says "Audio or Video File, up to 100

The error message doesn't provide any sort of insight into whether or not that's the case. Contrary to that, the upload box says "Audio or Video File, up to 100MB". So that would imply that they aren't accepting only video input. While I agree with your suggestion that the reason why they seem to be accepting only video files is based on watermarking the output, the upload box seems to accept an audio file input. They should say "We only accept video file input". It's possible they accept audio-only files for higher level paid plans where you can uncheck the watermark box in the Advanced options.