How "pinless" moisture meters work?
As it happens the SignalPathBlog recently looked at a different moisture detector. That one uses a capacitive sensor (just a plate of metal) and detects when it is near a conductive object which could be something that contains moisture.

This detection probably works using an oscillator circuit which would change frequency when the (coupled) capacity of the sensor plate changes.

Although the device you show looks very different from the FLIR device I expect that it works in a similar way. For both it is a relative measurement so it is only usefull to find wet and dry areas on the same material.

I also think this has nothing to do with RF, the steel rod an ball may look fancy but it does not look like an antenna to me.