I watched it a few times. I even found a YouTube that has the full broadcast, in case the interpreter was merely behind Biden's speaking.
She's not interpreting anything he's saying. She didn't fingerspell "George" or "Trump". It almost looks like she's signing from a pre-typed speech. She's talking about how important the vote is, and how he needs your support.
Thanks for the interpretation. Curious that she makes him look coherent to the hearing impaired.