Sorry, but Nasu does not have the right to retroactively change things and have everyone accept them even if they don't make sense.
Further, as I have said before, there is sufficient ambiguity there that what I said can be true and that, at the same time, it can also be true that Shirou "can't trace Excalibur". Because, what he made is not Excalibur, it's a massively-degraded copy of it. There is no reason why that statement cannot fit with what Nasu said.
It is ridiculous to use an overly-literal interpretation of what Nasu said that is in clear and direct contradiction with the contents of the actual Visual Novel, which takes priority in canon terms over the contents of some side-material book. The VN comes first, and the VN clearly shows him tracing Excalibur, or some degraded version of it with the same basic form and Excaliblast ability.