Yes...the automatic car washes do use high pressure jets to spray the undercarriage and it will indeed wash the fluid film off. The same with the self serve bays hand wands which are basically variable controlled power washers. It's possible the spot free rinse might not wash off the fluid film but then is it really getting the salt off?
Rain water splashing up on the undercarriage may remove some fluid film at a lesser reduced timeframe over many, many many miles but most of it should stay intact during the majority of the winter season thus keeping the salt brine from further rusting your vehicle... that's why it should be re-applied every season or two.
For someone who wants to wash their underbody during winter, they probably shouldn't waste their $$ on fluid film. IMO, fluid-film it and wait til spring to wash the underbody or don't fluid film.
Their might be videos on you-tube that explain do's and don'ts.
On a side note, you have to wonder, as we turn into a more green friendly nation/world, car washes are starting to recycle water, would you not be spraying recycled salt water onto your car???
Maybe we should just hose rinse/wash our cars at home like the old days.