> If a cell is said to be a sum of 7 underlying cells, then their sum of objects must equal to this cell's sum of objects, which is not the case in H3.

If you are using an H3 index as an index to the aggregate of the logically-contained cells at some specified level, it absolutely is true (in this case, the exact geographic border is the border of the set of contained cells ay thr specified level, not the border of the higher-level cell specified by the index.)