I think you can safely ignore this cease and desist. Just change the name and add a disclaimer so your users know that by using your extension they are violating their acceptable use policy.
This is no different than any user writing a Tampermonkey script to modify any website they want to modify. Even further, this is no different than a user opening the Dev Tools console and modifying things there.
This is no different than any user writing a Tampermonkey script to modify any website they want to modify. Even further, this is no different than a user opening the Dev Tools console and modifying things there.