You're right. Only for setup though, which is something I guess.

How to Use Signal WhatsApp Without Phone Number?

As mentioned above, a valid phone number that can receive calls and text messages is required to create and verify your Signal Account.

Since, your mobile device does not have a phone number or you do not want to use your phone number, you can use a Landline Phone Number or a Virtual Phone Number as provided by TextNow, Google Voice and others to verify your Signal Account.

Once the account verification process is completed, you will be able to use Signal on your mobile device, regardless of whether or not it has a phone number or SIM card installed on it.