There was a case where a doctor listed abortions as one of their offerings on their website, and that was seen as an advertisement. [Citation needed, my memory might fail me here]
So "no advertisement" pretty much means "don't tell anyone anywhere beforehand".
And that was seen as non-helpful, especially as abortions are legal in Germany.
Nope, the criminal code forbids abortion, but doesn't punish it in some circumstance (within some months of pregnancy, after a counselling session and some time for thinking it over etc.)
There are huge debates to make it more legal by turning this around or clearer restricting in criminal code.
§218a lists that it is not a crime under certain circumstances ("Tatbestand des § 218 ist nicht verwirklicht, wenn"), but the heading of that one is "Straflosigkeit", so "no punishment".
IANAL but this reads like it is not a crime under the listed conditions. Which is different from no punishment.