Not wrong, but open to misinterpretation.
The graph in the MI's for the heat exchanger show the flow when running at max output with a 20C differential. I would assume that 20C is the max differential allowed across the hex as any higher could introduce stress.
If the boiler is only producing 5kW but the flow rate is the same, the differential will be 1/6th (5/30). But if the hex can withstand a 20C differential at 30kW, surely it can do so at 5kW, which means a flow rate 1/6th of that at full output, i.e 203 litres/hr.
This is made clear by paragraph 3.14 in the MIs:
3.14 Pump specification
The pump should be fitted on the flow pipe from the boiler and have isolating valves each side. A variable duty pump should be set to give a temperature difference of no greater than 20°C.
The criteria is not that the flow does not drop below 1220 litres/hr but that the differential does not exceed 20C.