That is unless he manages to escape before that, which is fully likely.
This is just totally untrue.
St. Helena is over 1,000 miles from the nearest landmass, Africa. In terms of topography, it is almost entirely surrounded by rugged cliffs with few safe landing spots, which were being kept constant guard, as was Napoleon by a garrison of British soldiers.
Further to this, it wasn't on the shipping lanes, so any foreign ship noticed approaching the island would be treated with suspicion.
Now even if a man in his fifties-eighties did manage to either somehow get past a garrison of soldiers at the few exit points or scale a hundred feet of volcanic cliff face, there was a further obstacle. From fear of escape, a further garrison was placed on Accession Island, between St. Helena and Europe.
So, unless Napoleon was an SAS Commando style Action Man, there was no way he was escaping.
St. Helena was chosen for a reason. If Elba was jumping a small gorge, St. Helena was jumping the Grand Canyon. With weights attached to you.