I had a look at it here and found it uses around 1kWh of heat from the house + 0.2 kWh of heat via electricity, so a total of 1.2 kWh in heat (over ~7 minutes for me). 50l of water might be sufficient to provide that energy, say starting from 30 to 15 °C is around 0.9 kWh.
But you’re completely right, the flow rate is what breaks this as the buffer has insufficient volume to provide water for the whole defrost cycle. When the volumizer is in the return path, this will be used to provide the energy for the defrost but the cold water will then directly reenter the radiators. It might be even beneficial to have the buffer in the flow path as the incoming cold water will be mixed with the warm water in the buffer and what enters the radiators is at least somewhat warmed up.
I’m really looking forward to the first really cold days to test out my idea to just close all my TRVs when the defrost starts (can detect this and close them wirelessly), have the automatic bleeder valve open and my 100l volumizer deal with the defrost, only to reopen the TRVs once flow temperature is at a comfortable level.