This lump sum is a tax allowance for those who receive money for their voluntary work.
Most volunteers do not receive any expense allowances at all. This is also the case for the leaders of the self-help groups in the field of IBD in Berlin and Brandenburg. They are people who are affected and who work for free. Therefore, they cannot claim this tax allowance at all.
Here is a link to some information on the honorary lump-sum.
You can read more about it here.