While it was entirely possible that Locke's prediction would prove to be incorrect. It was not 'dumb luck' it was an 'educated guess'. 'dumb luck' literally is pretty much exactly the opposite of what occurred here.
But I guess someone actually gaining sight just wants to be done ASAP with those silly experiments and go to the theater ;-)
It should be possible, if the subject is given enough time and approaches the task from a rigorous angle, to deduce that the object must be a cube. through a path like...
eyes -> visual processing of the brain -> logical facts about -> mental model of the object in question -> categorizing the mental model -> determining what the object is.
However, I think, the fundamental issue is that in sight, touch etc, typically the concious/logical part of the brain is not involved in determining objects. probably because this is much to slow. There is dedicated hardware/software that does this as far as i am aware.
There are people with the condition where they cannot name, or identify objects from touch but can from sight, even more interesting some people can identify things from touch with their right hand, but not with their left.
Rigorous logical reasoning requires some axioms, and someone who has just gained his or her sight has no reasonable axioms to apply (or, rather, no way to decide, even on the basis of experience, which axioms are reasonable to impose). Suppose that I felt an object and discovered that it felt the same from all angles. Could I conclude that it tasted or smelled the same from all angles? I don't think that is a reasonable conclusion. Why then should I conclude, with no prior evidence to guide me, that it would look the same from all angles?
You can't 100% assume it.
But if there's no reason to think it's a trick question, and one object does smell and taste evenly, while the other object has multiple distinct smells/tastes...
You can get a pretty good idea.