Hi Nicolas,
Yes, when you create a Firebase project, an additional Server Key is created for you. If you check the credentials for the project in the
Google developer console, you can see that this key has an '(auto created by Google Service)' label.
Both keys work with their Sender ID; it's up to you to replace the key in your code or keep using the old one. Whatever you end up doing, I'd recommend not to delete the key generated by FCM, I fear there could be unforeseen side-effects.
I'll add something to the doc to better explain this. Thanks for flagging it!