Finlandīs pre-1939 borders never went as far east as "half way between Lake Ladoga and Lake Onega", so the intercept definitely took place in genuine Russian airspace. Only about one third of Lake Ladoga used to belong to Finland before WW II. See this map, where areas annexed by the USSR after the war are marked in dark green:
http://www.plappi.fi/kunnat/salla/ky...n_kartta_1939/