I can't unpair because I don't have the phone
How do I unpair without the phone? Your instructions that say without the phone still have the phone.
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Unpairing without an phone is literally not possible, because a key part of the unpair process is an automatic backup of the watch to the iPhone.
But you can still reset and erase everything using the watch alone, as described on that page.
Note that you can also reset your watch from icloud.com as well.