Questions regarding the validation rule "Input With Missing Required Fields"

[glen-84]

The spec states:

Let requiredFields be the intersection of all field names across all input objects ...

But the invalid example shows title in one input and not the other, so the intersection would surely exclude title?

Is it supposed to be the union of field names?

Second question: Must the input field be non-nullable in the other schemas, or any nullability? The spec mentions:

If requiredField is not in input:

It doesn't say that the field in input must be non-nullable.

A third example would likely help to clarify this.