I went to Tredzbiked and also found the Marathon Plus in a 28” and I couldn’t find any ERTRO/ISO size for the tire.
It was listed under Hybrid Bike Tyres.
They also have the same tire in a 700c listed in that category. However, if someone just searches by 28”, you never see the 700c.
The OP has a valid complaint if the seller doesn’t provide the ERTRO/ISO size. This doesn’t change anything, but I understand the frustration.
John