This feature is a very good step forward in security. I will take a look and if the implementation is sound, I am going to use Tailscale (namely if the Tailscale is compromised, I will not be automatically compromised, unless I manually accept external public keys, or install a bad update).
The problem with malicious updates can be addressed by providing as easy way to check the code signature. With a standalone infrequently updated app such as an AppImage app, this can be easily done by verifying the GPG signature upon download.