Second, at the time it was passed, car carry was illegal without a license, so it makes no sense to claim that it was intended to give general notice to those carrying without a license.
You wouldn't be claiming the 30.06 language was passed with that intent. The point is that Section 30.05 doesn't require any specific form of written communication. If you have notice, via written communication, that entry with a concealed handgun is prohibited, then refusing to leave or continuing to enter is criminal trespass with a firearm (a Class A misdemeanor).
You would simply be claiming that the 30.06 language satisfies the "Written communication" that the concealed carry of handguns is a condition that prohibits entry under Sec. 30.05. You aren't making any claims about the drafting intent.
The counterargument, as you have pointed out, is that the sign very specifically prohibits concealed carry of a handgun by a "concealed handgun license holder." This raises the question of whether a reasonable person would read the 30.06 language and conclude that concealed carry by a non-licensed holder in their vehicle was OK. At best, that is a legal gray area that could very easily bite you if you were ever unfortunate enough to be charged.
From the risk-avoidance perspective, the safest thing to do is treat a 30.06 sign in a parkng lot as notice to people carrying in their cars without a license.
First of all, it's extremely specific. Both the carefully specified sign and the wording of the law apply exclusively to license holders.
Yes, it is specific; because as you pointed out, at the time it was written, there was no way for a private citizen to legally conceal carry a handgun without the license. I don't think that is an aspect of the legislative history that works in your favor in court since it means at the time the law was written, the sign was effective notice to anyone legally carrying a concealed handgun.
From a practical perspective, it is an area of law that is unlikely to ever get fleshed out though since it only applies in a very narrow circumstance and your chances of getting caught are very minimal, and even after getting caught, you'd still have a decent chance of not getting charged.