You have to long-press that icon, not just tap it. Granted, it enables lockdown too, which is what the physical button used to do. As other have pointed, you can also do that by long-pressing lock + volume up.

It is still ludicrous how Apple had to work around its own decision to not use the camera button for anything else, since allowing us to move Siri to the camera button would leave the lock button for the easy duress gesture it traditionally has been.