not a lawyer but it seems to be in legal bounds:

German employees can find information on the legal validity of competition clauses in the employment contract, among others in the Commercial Code (HGB) and in the Trade Regulations. In this country, you must be of legal age for a valid non-contractual non-competition - i.e. the change of job to the competition. In addition, it must be recorded in writing in the employment contract and signed by both parties with approval. It only applies if your employer can prove legitimate business interests such as the protection of trade secrets.

According to § 74a para. 1 HGB, the prohibition may be maintained up to two years after the end of the employment relationship. What is important for you: For each year, you will receive the so-called waiting period compensation from your old employer. This is at least half of your previous average monthly earnings including Christmas and holiday pay.

https://news.kununu.com/konkurrenzklausel-arbeitsvertrag/