The author apparently expects the safety number to change in order to alert the person on the other end that there "might be a hostage situation," evidently not realizing that the attacker could just, well, use the unlocked phone right in front of them.
So from that point of view it would be legitimate to argue that I might want to get notified if one of my contacts transfers his account. I can then double check : “Did you just transfer your signal account to a new device or was that an attacker?”
That might only be interesting for high-risk users though and could impair the UX. Why not make it optional?
The temporary access threat model is a common criticism that people use, but it is largely incoherent. Once you are making human judgements like "enough time to transfer a signal account but not enough time to install a rootkit" things quickly break down into meaninglessness.
There are many cases where an attacker can access a device for a short time and/or without the owner realizing that the phone was tampered with.
If you leave cryptographic keys lying around unprotected they should be assumed to be compromised.
This is what you originally responded to. I paraphrased it. The "heavy lifting" meme that you've employed is rarely more than a shallow dismissal. Be better.