I imagine it is a conservative estimate. We'll never know how many Russians died fighting for sure and I imagine it'd be the same for the Holocaust. Such a wide area to cover and in territories with long memories.
Like the priest says, wherever there were Jews, the Nazis weren't going to stop. They even tallied up the number in the UK if I'm not mistaken. Such a massive thing to get your head around though all the same -mass graves, ravines, camps, forests, woods...and not just by the Einsatzgruppen themselves but locals as well.
A noble endeavour by the priest and an emotionally heavy one.