True, to a point. But the vans do exist, there just aren't very many of them by all accounts. They detect the oscillating frequency, so in theory it doesn't matter what type of TV you have or even what sort of connection. At some point a tuner will be involved and it's that little bugger that the detector can pick up.
I think what they actually do is look at the database of licence payers and monitor houses where people don't have one, rather than drive around with a radar.