Everything I read says this is an ADDED TAX, not a redirection of your tax (like the check-box for public campaign financing on a US Income Tax form is a redirection of some of your tax).
I think you are the one who is ignorant of the issue here... either that or all the Germans quoted in all the articles I have read (who live there and actually have to pay the tax) are wrong.
I’m betting that it is you who is wrong, not the German taxpayers.
If I were an anti-cleric I’d devise a system where the church becomes dependent on state funds and force people to supply those funds by paying an additional tax if they declare themselves members.
You can see several benefits. First you discourage people from becoming members. Second you generate resentment among some members who would not resent freely giving on there own. Third you remove as much feeling of generosity as possible from those who give. The benefits for those who dislike religion just seem to go on and on.
I don’t know if you reduce your tax bill by renouncing church membership, but that choice seems to be the most important one for practical, social engineering consequences.