I do believe that Baptism of Desire for anyone other than catechumens, or those at least with explicit faith in the Holy Trinity and Incarnation, is heretical ... OBJECTIVELY SPEAKING.
This has not, however, been explicitly condemned as such by the Church, and so a person would not be a formal heretic for believing in it.
So, for instance, it's always been dogma that Our Lady was immaculately conceived, and so the position of St. Thomas Aquinas was OBJECTIVELY heretical. Yet it was not an act of formal heresy for him (or others) to embrace it due to lack of ecclesiastical definition.
I am arguing that BoD for anyone other than catechumens is heretical in the material/objective sense, not in the formal sense. I make arguments from defined Church dogma and doctrine to make my case.
But, and this is where the Dimonds are mistaken, I can put together the most solid airtight syllogism anyone has ever seen, but the logic is still based on my private judgment. There can always be some missing distinction, and it's up to the Church to define or condemn things.
So, for instance, if I were a priest, and someone had a loose understanding of BoD, I might try to correct him, but I would not refuse him the Sacraments on that account.