The readme on the linked github reads: “MyShell reserves the ability to detect whether an audio is generated by OpenVoice, no matter whether the watermark is added or not.”

As you say, right at the bottom https://github.com/myshell-ai/OpenVoice

Ah, thank you. Guess that's OK that the company/service do whatever they want, the paper/technique doesn't involve watermarks, so it'd be easy to remove/modify whatever they do in the library/service itself.