They use a magic number, rather than a digest derived from the schema[1], but otherwise they do as you suggest. The magic number is given to the signing function (sender side) and the validation function (receiver side) but does not increase the size of the transmitted message.

[1]

I think that's what you mean by digest, but maybe you just mean `type` = `magic number`

The problem here is that a digest derived from the schema would just reintroduce the possibility of confusion of identically encoded but semantically distinct types.