That's what I thought. I asked a Fi rep and she emailed me this reply:
Thanks for reaching out to Project Fi support! My name is Sue and I'll be your Project Fi Specialist today! I can definitely help you out with your question about switching Sim Cards.
This is a great question! We are excited to welcome you to the Project Fi Family and we are excited we are able to offer you our great service. When you purchase a device each device will come with a Sim Card after the Sim card is activated it will only work with that phone, you will not be able to swap the sim cards, in the event you do swap the Sim card it will automatically deactivate the Sim card. Sorry for this inconvenience, I know back in the day you use to be able to put your Sim card in any phone and it would work, but it doesn't work like that anymore. I guess it's a good security feature also.
If you have any other questions, you can contact the Project Fi team directly anytime at 1-844-Talk2Fi. You can also reply to this email, and we'll get back to you as soon as possible! It was my pleasure to assist you.
Thanks!
I'm guessing she thought I wanted to activate a second sim card?