6 nails are for high wind areas it’s true, but the most important component in resisting wind is the adhesive strip. If the bond is broken and the wind gets under the tabs the shingle may or may not pull over the nail heads, but the tabs can still crease and tear. Manufacturers are serious about their warranties. If 60 MPH max shingles are installed in a 90MPH wind zone, I’ll bet a lot of them are not going to stand behind the warranty.
It’s so common, I don’t recommend replacement, but if there’s a chance the warranty is in effect, I think the buyer deserves to be made aware of that condition.