I think he was referring to the fact that since Siam no longer exists, you can no longer have a set of 'Siamese' Twins, since it's no longer physically possible to be born in Siam. Of course, the whole point is moot as obviously only one known set of 'Siamese' twins were born in Siam anyway, making the entire term a misnomer.

'Physically impossible' was still a poor choice of wording, however.