Uhm. I think you misunderstand. An Invisible Solid Fog will be invisible, therefore everyone would normally be able to see through it.
A person with True Sight/See Invis., however, would negate the Invisible part of the Solid Fog, and would be able to see the fog.
It's great for, say, a Rogue. He casts Invisible Solid Fog, hides, and hopes the Wizard activates True Sight