Unfortunately, this is the world Signal lives in. For binary debian packages to be installed securely directly from a vendor requires the installation of gpg keys which is what 2 of the 3 commands are regarding. If Ubuntu had spent resources to develop a convenient way for developers to directly provide binaries to the users of their OS instead of developing a system where they are gatekeepers and distribute all packages Signal would now be able to provide an easier secure method of installation. If you were providing a privacy focused product which in some uses that privacy can be the difference between life and death, would you want to turn over supply chain protection of that product to a 3rd party?