No, it's not okay to use Zoom (or any other method of communication outside UpWork) until you have accepted a contract. After that, you can use whatever you want.
I think the issue here is they haven't updated the chatbot with the new terms of service that came in on June 26. I just tried searching "share email with client" in the chatbot and it gave me the following reply, which is out of date:
Upwork Messages supports text, voice, and video communication, but if you need to take the conversation off Upwork, you can. We simply ask that you not share external information on your profile, proposals, or job posts.
It needs to be updated
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