The energy certificate unfortunately only relies on the demand without actual consumption. However, this is prescribed for old buildings so that the demand must be determined. But since no one can look into the roof and the residents apparently know little, it only has a thin data basis and is therefore not very reliable.
How many people live in a hut is pretty irrelevant for the heating demand without hot water. Therefore, the information from suppliers about "family with two children and two adults" are only average values and even less reliable than the energy certificate: if the hunter once shoots to the left and then to the right, the hare is dead on average.
The consumption naturally depends on the user, as already mentioned. But if the current residents did not consider the topic that important, I would not assume that they kept the house NOT half cold in winter to save energy.
By the way, the window construction year is on 99% of the windows. On the edge of the glass within the spacer between the panes (the silver running thing around the edge of the pane), there is always some information somewhere. Sometimes manufacturer and date and U-value all spelled out. Sometimes only "7/92" or something like that. That is the manufacturing date. Since nothing about the windows is mentioned in the energy certificate, I would initially assume that they are from more recent years (after 1994). If that is the case, then everything is fine. Otherwise, they can be replaced, or only the glass depending on the type/condition.
The cellar looks like the construction year suggests. Rammed earth on the floor and the walls built from some kind of stones. Since I am not from the area, I do not know which material was common back then. A horizontal barrier will not have been installed. So either leave it as is or plan with expertise if the cellar is supposed to be more than a slightly damp storage room.