To understand why the expression: [ [], [[]], [[[]]], [[[[]]]], [[[[[ ]]]]] ] has type [[[[[[a]]]]]], you need to understand what is the type of [] first.
Actually, [] is equivalent to [a'], where a' is type variable without any constraint. That means a' can be any type. So, when a list is constructed, say, [ [], [[]] ], it is legal to replace a' with [a] as:
[ [], [[]] ] ~ [ [a'] [[a]] ] ~ [ [[a]], [[a]] ] ~ [ [[a]] ]The same reason why
[ [], [[]], [[[]]], [[[[]]]], [[[[[ ]]]]] ] ~ [ [[[[[a]]]]] ]But when you constructs a list as [ [True], [[]] ], there is no way to convert True to a list, even though [a] can be any type of list. The same reason why [ [True], [[]], [[[]]], [[[[]]]], [[[[[]]]]] ] is illegal.