Okay first of all, having seen the amount of damage on real archaeological finds there's no way an aluminum can survives for several centuries without disintegrating. In fact, it seems to be expected to take between 200 and 500 years.
But second, there's no way that shape would be called a shield. Even if some archaeologist was silly enough to make that error there would be dozens of contemporary HBMA practitioners pouncing on the poor bastard with their arguments about practicality and various attempts at recreation and demonstration.
Unless this is assuming a sample with one half of the can worn away and the other preserved in some miraculously half-protective condition. Though I'd really expect the top and bottom to outlast the wall.