Suppressing testimony by punishing people for speaking honestly (nor merely disregarding their testimony) undermines the very foundation of thr legal system.
This is obviously wrong. Lying is broadly protected by free speech but firmly illegal, not just inadmissible, in court testimony. Courts also have the ability to compel testimony under criminal penalty, while free speech includes the right to say nothing or dissemble. Evidentiary standards place further requirements even on the source of what you are saying, regardless of its truth.
Or as the 6th Circuit put it, "The courtroom is a nonpublic forum, where the First Amendment rights of everyone (attorneys included) are at their constitutional nadir."
Even if he's 100% correct, that doesn't mean he has a first amendment right to testify!
"The yellow light timing is below table X of the traffic code, and therefore my ticket for running the red light is invalid because I was not given the chance to stop in time."
That's an argument that an individual can straightforwardly make and a judge can choose to accept or not by following the same reasoning. It involves engineering analysis, yet it is low stakes enough to make hiring a Professional Engineer prohibitive.
Under the North Carolina board's position, a person who makes this argument is committing a crime. That effectively creates a privileged class of people whose decisions (or oversights) cannot be questioned even when they're blatantly wrong - in this example, the traffic engineers responsible for the intersection.
And if you attempt to provide expert testimony in your own defense, the court will likely laugh at you. You will have to hire an outside expert to provide expert testimony.