coerce fails for Coercible type families

I want to bring attention to this reddit post that boils down to

type family   X a
type instance X Int  = String
type instance X Bool = String

data T a = T (X a)

but not being able to coerce

coerce :: T Int -> T Bool

This gives the error “Couldn't match type ‘Int’ with ‘Bool’ arising from a use of ‘coerce’”.

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