uh, no she's not. i'm not saying no Wiccan anywhere doesn't worship Rhea, just that Gardner never said 'we worship Rhea'. [then again, i'm not BTW, but every book i've read on Wicca and all my interactions with the Goddess i've never come across the name Rhea]
The Goddess is the Goddess, and the God is the God. you can argue they're based off other deities, or perhaps all deities are branches of them, but The Triple Goddess and Rhea are two different beings. it doesn't mean you can't worship both, or incorporate Rhea into your Wiccan path, but regarding BTW and the original Gardenian path, no, it's The Triple Goddess and The Horned God and no-one else.