Is a granny flat necessarily an (own) residential unit?

The building desire of the OP does not seem to be all that serious. If he has been searching for 10 years and it now only fails because he cannot secure a funding amount, then he can probably indeed leave the field to serious interested parties.

Post from Feb. 2013

(Emphasis by me)
Hm, is there also a funding option for (manager) apartments in commercial buildings? Specifically it's about Schleswig-Holstein, if that matters...

The money did not seem to be available back then and still isn't now, to build.
It is sad that so much space is given to hunting in life, which then consists more of frustration than bliss. It's a pity that the original poster cannot get out of the hamster wheel.

The building wish of the OP does not seem to be very serious. If he has been searching for 10 years [...]

... I would seriously consider rightsizing the search requirements to "only" one residential unit for the building couple themselves, instead of simultaneously wanting to switch to the landlord camp (in the anyway non-profitable lot size of 1 residential unit).

[...] and it now only fails because he cannot claim a funding amount, then he can probably actually leave the field to serious interested parties.

The KfW funding product "Bonus for a second residential unit" aims to accelerate the change in housing occupancy by tenants from energetically outdated to energetically modern living space. It is not an economic stimulus program to support the "self-employed small landlord".

Is there a regulation according to which a small (clearly subordinate) granny flat (especially for later care in old age) is mandatorily permitted or at least can be 'argued for,' or [...]

The answer option "or" is correct. In multi-family residential construction, you don’t get a condominium offered in a double pack with a second apartment (not even a small one as a bonus). The "granny flat" is (conceptually as well as culturally) a relic from the 1980s, before the relevant purposes moved in the Income Tax Act from house number 7b to house number 10e. In a certain sense, it is the still living sister of the mullet haircut.

... why an "angry" reaction to ? This is the reaction to

Is there a regulation according to which a small (clearly subordinate) granny flat (especially for later care in old age) is necessarily permitted or at least 'argued for'

Residential units paid for by all our tax money should be promoted, not eventualities.

Since you want an official EWL because of the funding, the topic of parking spaces also comes up. An EWL generally generates at least one additional parking space, and on a semi-detached house plot, that is usually impossible.