"Baptism of Desire which is a perfect act of charity that includes at least implicitly the desire of Baptism by water" - Moral Theology, 1949
The Baptism of Desire is the Desire (votum) for the Baptism of Water. Agreed?
It follows that an "invincible ignorant" or someone in the blindness of false religion cannot have it.
How could an invincible ignorant (hypothetical native, anonymous Christian, benevolent non-Catholic relative, etc) have an actual desire (votum) to receive sacramental Baptism if he does not know of the need or existence of the sacrament in the first place?