Well I'd believe someone who was there.
They are mistaken.
"...the City of Houston has an ordnance banning the carrying of guns in the George Brown Convention Center."
By TX law, the City of Houston can not ban carry in that building.
https://www.dps.texas.gov/section/handgun-licensing/faq/laws-relate-carrying-handgun-faqs
13. Does a state agency or political subdivision (e.g., a county or city) have the right to exclude license holders from carrying concealed handguns on state, county, or city property?
No. State agencies and political subdivisions cannot use
§30.06, or §30.07, Texas Penal Code, to prohibit handguns from their government property. If a state agency or political subdivision unlawfully posts a sign under §30.06 or §30.07, the state agency or political subdivision can be fined $1,000-$1,500 for the first offense and $10,000-$10,500 for each subsequent offense.
There are a few exceptions to this law, but they apply to things like courthouses and police stations and certain governmental meetings.
Here's the law.
https://statutes.capitol.texas.gov/Docs/GV/htm/GV.411.htm#411.209
Except as provided by Subsection (i), a state agency or a political subdivision of the state may not take any action, including an action consisting of the provision of notice by a communication described by Section
30.06 or
30.07, Penal Code, that states or implies that a license holder who is carrying a handgun under the authority of this subchapter is prohibited from entering or remaining on a premises or other place owned or leased by the governmental entity unless license holders are prohibited from carrying a handgun on the premises or other place by Section
46.03, Penal Code, or other law.
(b) A state agency or a political subdivision of the state that violates Subsection (a) is liable for a civil penalty of:
(1) not less than $1,000 and not more than $1,500 for the first violation; and
(2) not less than $10,000 and not more than $10,500 for the second or a subsequent violation.
(c) Each day of a continuing violation of Subsection (a) constitutes a separate violation.