The percentage of these is so low as to be insignificant. The occasional anomaly does not warrant the number of those who believe they were ‘born with the wrong body’. Chimeras exist but are astronomically rare. And, as you point out, they were two people who ‘accidentally’ merged. Not one person (originally).
Legally and practically they are one person. They are not surgically separable, unlike, say, some Siamese twins.