Threema runs its own servers in switzerland- they got nothing to do with american clouds!
A "unique Identifier" is something that you can link to a person!
Everybody can use Threema without personal informations - no mobilenumber or email needed.
If a User does that, there is no way to find out who is behind a Threema-ID!
Furthermore: Threema don´t log anything - no metadata - no IPs!
All together: More privacy & dataprotection isn´t possible!