I don't see how you got "as if it meant WD40 traps moisture" from that. Most people are familiar with WD40 so I made the comparison as both products displace moisture. I assume your question refers to the rust spray. To my knowledge, it does not dissolve any other coatings. It is meant to act as a sealant but has the property that it spreads across adjacent surfaces. It's a light oil so it can creep a fair distance. It's possible, though, that if the vehicle had a hard undercoating applied, the rust spray could creep into any cracks and get between the hard undercoating and metal, which might cause the hard undercoating to separate. I don't think anyone would recommend using both on a vehicle.
WD40 is a penetrating oil and I think it is more aggressive chemically. I would test it on small non-metalic areas first to make sure it was safe to use.
Another benefit from oil sprays is that they keep the nuts and bolts from rusting up completely. A car that has been treated regularly can be easier to maintain because you don't need to take a torch to rusted up bolts for disassembly. You can ask your local mechanic about that but my mechanic sees the difference and time is money.