I have no idea what that note was trying to tell you. It doesn't make any sense. Perhaps it is a reference to possibly gouging the sides of the kettle. Scratches and gouges would expose raw unpassivated metal to your wort possibly causing off flavors. I think that's a stretch, but maybe that was their intent.
The FB for the Polarware kettle is a close fit and you will understand the above after installing and removing it a few times. I wouldn't worry about that note at all and just take it easy when putting it in.
The FB is primarily intended for use in a mash tun. It also works well as a strainer in the boil kettle. It will hold back whole hops and some of the debris from pellet hops. A significant amount of the pellet hop debris will pass right through a false bottom, but it won't do any harm in your fermenter so you are good to go it seems.