I don't know what it is that makes something sentient, but I do know what makes something not sentient, and that's lack of self-direction. This software doesn't start talking whenever it wants; it only responds when spoken to. It doesn't ever refuse to talk, it always replies. It doesn't ever not know what to say. It's never not in the mood because it's having a bad day. This isn't because it's polite and eager to please. It's because it is a function that takes an input, transforms it, and spits out an output.

The only reason it's different from any other function is because the output is nicely stated prose that causes emotional reactions from the reader. If all did was output an integer, we wouldn't be having this conversation. But because humans have a deep sentimental attachment to language, we are fascinated by something that can produce it.