They didn't remove the employers ability to ban guns in the workplace, they trimmed it back so people could travel from their homes and work in possession of firearms. Now if they had told the employer they cannot for ban guns from the workplace then the employer should not be liable for the things which occur because of that.
The parking lot attached to the building where you work IS the workplace.
If you trip and fall on the parking lot, you would sue for workman's compensation.
The legislature removed the employer's right to set policy, but did not remove their liability.
By the way, your right to drive to and from work with a loaded gun in your car was never impaired by the employer's policy of not allowing guns in their parking lot, you simply had to park elsewhere.
But these guys in Oklahoma, apparently too fat and lazy to walk an additional thirty yards in defense of their Second Amendment rights, and being under the false impression that they have a right to park on the employer's property, opted to use the force of government to get legal entitlement to set policy on other people's property.
Similar behavior is exhibited by leftist tree huggers and assorted environmentalist nutjobs.