How much does a cup of hazelnuts weigh

I can't say which things are actually used, but this would seem to confirm my above guess, along with the other one - you can use more chemicals in industry than you would in a home.

Product and process of blanching nuts

A product and process of removing the skins from nuts involves wetting the nut kernels with an alkaline solution and then with a peroxygen solution. The two substances react underneath the skin to liberate gas between the nut meat and the skin. The gas bubbles blister the skin, causing it to become loosened. Slight mechanical brushing is used to dislodge the blistered skins from the nut meats.

Method and apparatus for blanching nuts

Two counter rotating rollers are provided having elastomeric exterior coatings into which ribs and grooves are formed for intermeshing to blanch nuts at a pinch point therebetween. Grooves in the lower roller hold nuts as ribs on the upper roller pass across the nuts to break skins on the nuts. The elastomeric exterior coatings have surface hardness values which measure around 30 durometer. The squeeze at the pinch point between the two rollers can be repeatably adjusted for running different sizes of nuts.

Not really helpful for doing it at home. Hard to say which ones are commonly used in industry, of course. You can find a lot more patents along these lines.

I actually measured and weighed a cup of Nutella and I got 290g. Of course we have to take into consideration my 1 cup measurement (I'm pretty sure they are not exactly the same) and my scale. But it just shows that the 300g mark is not far off.