They are very real in the fact that they exist. They do not and can not pinpoint a premises that is operating a television without a license. They existed as a visual and promotional deterrent to persuade folk that there's every chance that they would be detected if they had a television and no license. It was in the BBC's interest to persuade the public, technically minded or not, that the vans did what they claimed and conveniently failed to mention that the knock on an unlicensed household's door was informed by a list of addresses against which there was no record of a current TV license purchase.
i have little doubt that some, perhaps even the majority of vans were dummies for deterrence purposes.
Nevertheless, I have sat the through the lecture from the ex GPO engineer who addressed our Amateur Radio Club, and examined the vehicle he had restored at very close quarters. I have no doubt whatsoever having examined this equipment that I, or any other halfway competent Radio Amateur/electronics engineer, could successfully use this kit to detect spurious VFO emissions from a TV tuner, and if the range is not too great could locate the room and floor, never mind the building, that the set was located in.
Obviously with modern digital conversion the technique won't work.
