78 years is not that far ago in terms of human generations, the children of the people displaced are still alive, and they've disputed it all the time since then.
Don't understand why you think 78 years is a long time, even less why you think it's equivalent to 1300 years.
The problem is that you assert 78 years is soon enough for the Palastinian claim to the land to still matter but 1300 years is too long for the Byzantine claim to still matter so we have a paradox of the heap situation where you have to choose a hard cutoff time period after which all claims to land are invalid.
There's no need to be technical about a social aspect of human life, there's no paradox of the heap when considering the human aspect: people alive have felt in their skin and soul the displacement of their parents which is not true at all for a claim 1300 years old.
A historical event from 1300 years ago is a bit irrelevant to contemporary issues.