First of all, the number of confirmed lost bitcoin is far lower than the amount we are estimating as lost. So, you don't know whether people are not touching their bitcoin for long time or they have lost the access.
And the result can not be correct because you don't know which coins belong to which person/institution exactly unless they publicly announced it.
We can't exactly get a data so we use assumptions on that part because inactivity could mean anything. To me, it doesn't matter whether there are a lot or there are less bitcoin that ordinary people like me can get because getting even to 1 BTC is a difficult thing to do so we don't have to scramble over what to do with it because with the current numbers given by OP, I think that there is still a lot of bitcoin for every ordinary people.