I think that is incorrect information. The decisive factor should be
Zweckverband Abwassergruppe Dübener Heide, Bad Düben Landkreis Nordsachsen
"Wastewater and Fee Statute" dated 19.4.2012, page 11
§ 25 Fee scale for stormwater disposal (1) The wastewater fee for the partial service stormwater disposal is assessed according to the amount of stormwater that falls on the property connected to the public wastewater facilities and is discharged into the public wastewater facilities. Properties that are dedicated to public traffic in accordance with § 2 SächsStrG are exempt from the wastewater fee for the partial service of stormwater disposal.
The basis for the wastewater fee for the partial service of stormwater disposal is the sealed property area and the type of sealing. The sealed property areas of a property (§ 26 para. 1) are multiplied by a factor set according to the degree of water permeability and evaporation for the individual types of sealing (§ 26 para. 2). ...
§ 26
... If it is demonstrably the case in individual instances that not all the stormwater from the area underlying the fee calculation (paragraphs 1 and 2) is discharged into the public wastewater system in an allowable manner, the wastewater fee shall be appropriately reduced upon written application by the property owner, the leaseholder, or otherwise legally authorized person for structural use in individual cases. In doing so, the sealed property areas that are wholly or partially, permanently or temporarily not drained into the public wastewater system must be taken into account. § 24 para. 4 applies accordingly.
The property is presumably within their area of jurisdiction, isn't it?