Signal doesn't really care about identity at all, it leaves it up to the users to decide if "Steve" in their contacts is who they thought it should be, if they're happy to accept that without proof or if they've verified it was who they expected in person or out of band.
Modern Signal lets users put together a profile, like a Twitter profile, and like the Twitter profile you might know somebody whose profile name is "Grim Reaper" and whose profile photo is the Discworld Death, without you believing that is their real name or appearance. Maybe you decide that's enough reason not to mark your friend Suzie ("Grim Reaper") as Verified in Signal. Most likely not. Other Signal users aren't informed of this decision and Signal itself doesn't know what you decided.
But it does default bind your contacts to Signal users based on a telephone number they've proved control of at some point. So if you don't verify anything, a message from you to "Steve" could be received by somebody who registered the phone number you've associated with the contact "Steve". Signal's creators rationalise that this is what an ordinary phone user expects to happen.
If it's important to you that "SIM Swap" isn't used to create an imposter Signal account with your phone number - a reasonable concern for some people, you can set a "Registration Lock PIN" for the phone number. Anybody else in the future who wants to use Signal with that telephone number will need the PIN or their registration fails.