Technically, it means that there is some external mechanism that prevents price from rising. Typically that means government intervention. Healthcare often sees the government set a price ceiling on services to ensure that poor citizens are not priced out of receiving healthcare, so a shortage of medical doctors is quite possible. A shortage of truck drivers is unlikely.
Under colloquial usage, as it is being used here, the term is meaningless. Under that definition, everything is always in "shortage". It is simply used as an attention grabbing mechanism to express some kind of emotion towards the subject.
Surely a transient shortage is possible due to licensing requirements. You could offer ten million dollars a year, but there are only so many people with a current CDL and finite capacity exists to grant new licenses.
The cool thing is that if we really did see ten million dollar per year offers, then most people wouldn't want to use the services of truckers anymore (too expensive), so you'd still have the right number of truckers, if not too many, among those currently able to drive, without the need for more. No need to wait on anyone else getting licensed.
You have done well to highlight why a technical shortage is defined the way it is.
The licensing requirements for being a truck driver just aren't that onerous. It's way way harder to become a doctor, lawyer, hell, even hairdresser in many states.
A shortage of doctors is quite likely as many jurisdictions place price ceilings on doctor services.
In fact, where I live, it is quite illegal for a doctor to accept any amount of payment from a patient. We as a society believe that people, no matter how rich or poor, should have equal access to healthcare, so a doctor giving priority to a billionaire who can offer a handsome reward to get his common cold looked at over someone suffering something much worse is considered abhorrent. As such, we rely on a mixture of first-come, first-serve and needs-based priority instead of a price-based mechanism.
Never heard of any attempts to place similar restrictions on truckers, though. If you want to try and charge a billion dollars per mile, go nuts!
Under colloquial usage, as it is being used here, the term is meaningless. Under that definition, everything is always in "shortage". It is simply used as an attention grabbing mechanism to express some kind of emotion towards the subject.