Actually you are partially true and the explanation could be given based on the type of PhD you are looking for. There are three types of PhD one can commonly find in Germany.
Structured Programs: Many universities in Germany offer structured PhD programs with specific application deadlines. These programs often have set start dates and application periods, which are typically announced on the program's website.
Individual Doctoral Positions: If you are seeking an individual PhD position (where you work directly under the supervision of a professor), there is more flexibility. Professors may accept inquiries and applications year-round, depending on their funding situation and current projects. It's advisable to check the professor's research group webpage or contact them directly to inquire about open positions, if there is any. BUT remember to get accepted, you need to come with really solid research plan.
Industrial Doctoral Positions: This often has a couple of partners including both universities and industries. Therefore, there is no set timeframe to apply. You need to be actively search on both university and industry's career portal/page to find the announcements.