Thats total BS, if the scoring is only slight, you can sand it out if you are willing to take the time. The brass thats bonded to the steel backing is like 3-4mm, you won't be removing anywhere near that amount of metal. As for tolerance, its all spring loaded, it will take up any wear or metal removed when you flatten the plates (which will only be a few thou anyway).
If the scoring is really deep you may indeed need new swash plates, but if they are shallow i would give it a go. Worst case, give it a try and see how long it takes to get the scratches removed, if they seem too deep for you to remove you may just want to get new plates.
When you replace these plates, you really should flat plate the rotating group too, it will only need a slight touch up but it really should be done if you put in new plates.
If you feel more comfortable just buying new plates thats also your choice as using sandpaper and glass really does take some time.......