Yes this is possible, and more important, very probable!
The motherboard has a lot more sensitive components as compared to a PSU.
Let's say that there was a momentary spike in voltage due to the storm. The PSU is rugged enough to handle this, whereas the motherboard is not. The result would be a fried motherboard and an unaffected PSU.
After all, a PSU is basically a step down transformer and an AC to DC rectifier along with voltage smoothing circuitry.
The motherboard has ICs, caps, traces, Northbridge, Southbridge, RAID circuitry, etc. to mention a few. Certainly more sophisticated than a PSU by a very wide margin.
I wouldn't even think of comparing a PSU with the complexicity and sensitivity of a motherboard.
Tell your insurance guy (or gal) that they are full of crap and to engage their brain before they put their mouth in gear!
(Feel free to forward this post to them).