In looking around the Colorado Bureau of Investigation web site, I can find only two possibilities that MAY have allowed him to be charged. (Note the word MAY)
18-12-105. Unlawfully carrying a concealed weapon - unlawful possession of weapons.
(1) A person commits a class 2 misdemeanor if such person knowingly and unlawfully:
(b) Carries a firearm concealed on or about his or her person; or
(Jump to annotation -- The words "about the person" means sufficiently close to the person to be readily accessible for immediate use. People in Interest of R.J.A., 38 Colo. App. 346, 556 P.2d 491 (1976).)
(c) Without legal authority, carries, brings, or has in such person's possession a firearm or any explosive, incendiary, or other dangerous device on the property of or within any building in which the chambers, galleries, or offices of the general assembly, or either house thereof, are located, or in which a legislative hearing or meeting is being or is to be conducted
, or in which the official office of any member, officer, or employee of the general assembly is located.
If this were the convention hotel, then the portion I highlighted under section C could possibly be used as a reason. If not the convention hotel but if several representatives were staying there and had arranged to meet, you could possibly stretch section C to cover that.
If they were in locked cases and unloaded, it would be a hard stretch to make the "sufficiently close to the person to be readily accessible for immediate use" annotation of section B applicable, but that may be what they are trying.
In any case, I hope he sues the tar out of them, wins huge and donates half of it to the NRA legal defense fund.