This is of course a really bad requirement. A lot of people in the world do not know English, and would thus be unable to let anyone know that they will mutually honor some sort of Interspecies Protective Treaty. Mostly, they would be unable to read any such treaty, or reply with the word "yes".
First create a requirement that all human being will pass. Then see if cetaceans do pass it.
English? Where did I say that?
However, communication verbal or nonverbal that conveys agreement to participate in a society is a requirement.
This kind of communication would need to happen either individually or representatives cetaceans would need to come forward who could vouch for others of their species. From there, we'd need to observe whether or not the agreement(s) were being followed. This starts to sound silly, doesn't it? But that's what being a member of a society entails. That's what being "human" or reasonably equivalent to oen entails.
I'm also not sure why they would need to agree to some kind of treaty with humans just to be spared our cruelty. There's no evidence they mean us harm, we have the ability to protect ourselves if they did, let's just leave them alone.
That's a strawman. The proposal was to give Dolphins "the same rights as humans".
Protection of the creatures from hunting and physical exploitation is sufficient and something normally agreed to through international treaties. Equating cetaceans with humans is unnecessary and illogical.
The proposal is not equating humans and cetaceans. It is stating that cetaceans are close enough to humans that we should respect their life and not treat them a property. One hopes that an alien species landing on Earth would feel the same about humans...
Humans have to make the first step and propose a first draft because of their current position of power. Unfortunately, cetaceans so far haven't taken time to try to learn how to communicate with humans. A treaty with cetaceans would be a display of pure goodwill, as opposed to e.g. peace treaties between mutually menacing powers.
Another interesting question is how many treaties are needed, since cetaceans are obviously no more unified than humans.