Yeah, I think you'll create a Google account as part of the Google Fi account creation process.

If that really bothers you, use a different MVNO. Some offer free trials, but even if not, it's not too bad to buy a month of service. My provider is Tello, and the minimum service that'll give you SMS is $5/month. If you're clever, you can probably also find a VOIP provider that does SMS for really cheap.

My point isn't that Google Fi specifically is what you should use, just that it's an example of a service that offers a free trial, so you can sign up for Signal for free.