You will have to link me to something corroborating that statement as I couldnt find anything like that. All my knowledge of the continuation war was that Finnland was very hands off, not wishing to fully engage and generally only capturing territory lost in the winter war. Hitler was generally exacerbated by the Finns reluctance to fully commit and push beyond their established borders (they did send troops into land never claimed by Finland in the north but this was the only incident I could find). Further the leningrad front was under German command including all Finnish forces and the Northern front, including German forces was under Finnish command.

And even if what you say is true and Mannerheim was heavily involved in the direct siege of Leningrad, rather than just cutting off Russian access to the Borth of the city, I'd be hesitant to call a siege of a major Russian city (it's old capital, bearing the name of the founder of the soviet state no less) a war crime in the same way as the deliberate targeting and murder of civilians is a war crime