I went to get a quote from the local detail guy to fix these light scratches from my knuckle armor. He said these scratches are unfixable because it's a matte finish, tried to cover them with a plastic film as a test. Is he correct saying that it's permanent damage?

Yes, you can't polish a matte finish. Only way to fix is repaint.

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JESUS...I would have never guessed. I gooogled it and found a few sources that said the same. Cant polish it or the matte finish would get polished out

The scratches can be fixed, however you will have to sacrifice the matte finish for gloss. Full scratch removal is dependent on the depth of scratch relative to the thickness of paint. Deeper scratches will not polish out without damaging the painted surface. Your options are to either live with the scratches, polish the surface to gloss to minimize/remove scratches, repaint in desired color/finish, or bonus: cover in tank pad to hide?? Hope this helps.