If it's a 4K TV there's no way they can see one pixel at a recommended sitting distance, as a 4K TV has more pixels than we have rods and cones... or they are sitting 17 inches from a 65" TV.

You don't find it noticeable beyond four feet. Hmmm. Try this. Mask around the dead pixel, leaving at least a 10" square area showing, and have them sit in their normal watching position and tell you where the dead pixel is. This removes the image edges as reference points and makes it more difficult for them to use their memory of its position relative to the edge to say where the dead pixel is.

Move the masking a few times to put the dead pixel in different visible places. Be sure to show them this area without the dead pixel showing at all. It'll be interesting if they see it.

If they can repeatedly find the pixel, they have a point. I predict they won't be able to see it, especially as you don't notice it beyond four feet. Even if they can see it, there's the manufacturer's policy.