For liability, you're correct, but WC is a different set of circumstances and rules.
And it's not just TX law at play here, Bears. Insurance companies have lobbied pretty hard to get language in place across the country which more or less defines WC as applying only to injuries sustained in the line of duty, with perhaps exceptions being given for things like firemen who also serve as volunteer firemen.
I think you'd agree that getting hurt in the company parking lot doesn't qualify as an IOD, so whether or not he clocked out becomes pretty important to the discussion.
If he was off-duty, even though he was out on the AOA at the time of his injury, the insurance company is going to fight covering this as a workers comp claim.
The reverse side of the coin are a couple cases from NY and MA, where employees were either killed or injured in drunk driving accidents while still on the clock thanks to buddy-punching. The fact that they were still on the clock cause them to be classified as IOD and eligible for workers comp...