This is the stuff of conspiracy theories. The German constitution is pretty specific when it comes to whether and how the government can limit fundamental rights.
Article 10 of the Basic Law says that "telecommunications privacy shall be inviolable" and that "restrictions may be ordered only pursuant to a law". Secret agreements are no laws, so they cannot give the government the right to enact restrictions on telecommunications privacy. This requires a law passed by the German parliament following normal procedures (such as the G10 Act).
Prior to 1991, there might have been a (weak) international law argument based on Germany's limited sovereignty, but since 1991, this doesn't work, either.
The links you give rely entirely on the assumption that secret agreements can have the force of law; they do not.
Article 10 of the Basic Law says that "telecommunications privacy shall be inviolable" and that "restrictions may be ordered only pursuant to a law". Secret agreements are no laws, so they cannot give the government the right to enact restrictions on telecommunications privacy. This requires a law passed by the German parliament following normal procedures (such as the G10 Act).
Prior to 1991, there might have been a (weak) international law argument based on Germany's limited sovereignty, but since 1991, this doesn't work, either.
The links you give rely entirely on the assumption that secret agreements can have the force of law; they do not.