Case law and blackletter law will obviously look very different in terms of their textual representation, style, formatting, etc... And this will be true even when they pertain to the same ideas and the same concepts.
To state the obvious, semantics is about the meaning of things, not about style and not about specific word choices or specific syntactical forms (although sometimes these carry meaning as well).
This is the bigger point. In my own university studies, there was a clear segmentation between the common law and statute, although they are certainly interrelated.
It’s also worth noting that the boundaries between cases and legislation were not absolute, there were areas of the cases ‘mainland’ that contained legislation.
My point on the style was that in addition to differences in purposes, they are also textually different, which can indeed bleed into semantics.
I think this very point you're trying to make would be more persuasive if the analysis had modelled the relationships that do exist between blackletter law and case law. As we have already discussed, text similarity may not suffice to reveal these relationships. And while these relationships don't always exist, when they do exist they are very strong.