I was answering it was possible to: Is it possible that God will still refuse him salvation? Will this person learn by surprise at death that he was not baptized and then, too bad for you?
We do not know what God knows, perhaps God saw through his outward sincerity as being false. Who knows? Because God made the sacrament a requirement, it is ultimately up to God to provide it - same as He provides it for everyone who has ever been and ever will be baptized.
But you are talking a hypothetical situation....someone not baptized who does not fall into mortal sin is imo, impossible.
I have mentioned before in a previous thread, in the life of St. Martin of Tours, some girl died unbaptized. The monks knew that she was not baptized. They kept her body in the monastery for six months and prayed to St. Martin. St. Martin showed up at the monastery, raised the girl to life, baptized her, and she died.
Where was the girl's soul during these six months? If she had baptism of desire, then why water baptism?