Rated 2 out of 5 stars

Why does this very simple add-on ask permission for "full, unrestricted access to Thunderbird and your computer" ?

Because the permissions model in Thunderbird is poor and it doesn't have enough supported add-on APIs to implement the functionality of this add-on without embedding an "experimental" API in it and the indicated permission is required whenever an "experimental" API is included in an add-on.