You can choose the meaning for yourself. As for me, anything less than 100% is very surprising. Anyone who does not sign now is choosing to be brave and stand against all of their coworkers.
Just like if 0 people post public angry messages on Twitter about the CEO, it is meaningless. But if 5 people post, it is meaningful.
Even if 100% sign this letter, it is meaningless. But if 5 people do not sign, it is meaningful.
I think you're overestimating pressure and desire for conformity in a startup like OpenAI. People are working hard and largely committed to the project. There are no black-clad priests going cube to cube ensuring ideological compliance. Nobody is compiling lists of who failed to sign. If that's what you're thinking, it may be a sign of your own biases and how you'd act in a similar situation.