That really depends on how it is made. It's not useful to make blanket statements like this when there can be a significant amount of deviation from what you expect to be true without it not being jam anymore.
I wrote my comment because I think that people are shocked when they actually see jam being made at home and how much sugar goes into a batch.
https://www.compoundchem.com/2014/09/22/what-makes-jam-set-t...
The question is what did you granpa use as a preservative? The preservative is going to be the difference between say: jam, marmalade, chutney, etc...