Sticking to the HVAC side of the problem. It all depends on how tight the crawlspace is. Ideally, the house, return ducting, air handler, and supply ducts are all air tight and you have basically a closed loop system.
If any of the ducts or house leak (and they all do), then you have an imbalance in the system. The question is how much.
For example with an leaky return ducting and tight supply ducts, the air handler is drawing air from a infinite source and pressurizing the house with a mixed amount of conditioned and unconditioned air.
If you have the opposite (tight return ducts, leaky supply ducts), you are drawing air from a closed volume and pumping it out into the atomosphere. This creates a vacuum within the house and you start pulling in air from every crack in the house.
So, unless the crawlspace is entirely sealed off, you probably have more leak sources than you’d probably see in a regularly ducted house. I think the guy was trying to take advantage that most crawlspace floors are at a constant temperature.
If there is no problem with trapped moisture in the crawlspace and your clients really wants to know how well the crawlspace/house/ducts are sealed, you can recommend that a HERS Rater do a duct and whole house leak test. With a little luck, the rater may also be able to determine the leak rate in the crawlspace.
In California, we have HERS rater testing the ducts in houses when someone does a changeout. The average duct leak rate in a older home is about 30%. In new construction, it’s probably in 5 to 6% range.