It seals because the head of a thru-hull fitting is concave or has a groove to capture sealant and also sealant is applied under the flange between it and the backing plate.
Sealant gets trapped in the concave part:
Sealant gets applied here too and trapped in the bevel near the threads:
If this guy working on your boat is asking questions like this perhaps he should not be working on your boat? Especially if he feels NPS & NPT are a safe match because he got "five" threads... Really makes you wonder why we pay so much for these "experts" to work on our five, six and seven figure toys...