How?
The garage would have a positive duty of care, not just a case of "our staff didn't push the car on top of you so we're not liable".
There would be a duty of care but not to the extent of making sure people are working properly. You simply cannot watch customers none stop, if customers make mistakes that does not make you negligent. There would be ridiculous amounts of litigation (more so) if that were true. You have a duty to other motorists whilst driving, it doesn't mean you have to check their brakes are working.
The staff would have a duty of care to customers but it would be unfair to impose on the staff a duty to make sure every customer is doing everything safely. The disclaimer would enforce that point.
I'm not saying i'm right, i honestly don't know. But remember, public liability insurance is not mandatory, i would take the risk
and cut costs down. Nevertheless, i agree, it probably wouldn't be viable, but not because of the insurance aspect.
Employers liability insurance?
Wouldn't be that high seen as the employees aren't working on the cars and are in no real danger.
Occupiers liability insurance?
That covers the state of the actual premises, not acts done whilst inside. As long as tools, lifts etc are maintained there would be no problem.