Maybe they’re talking about lightfastness, which is resistance to fading in light.

This is usually more of an issue when drawing or painting, and many cheap colored pencils for example will simply disappear if you keep them on the fridge too long. I don’t think any normal pen ink has this issue. But then again, this isn’t a normal pen, so…

That said, if they do fade, I’d be surprised if Pilot doesn’t clearly specify this somewhere.