Under those rules, drow darkvision and Goggles of Night also become brokenly powerful. They can attack you and you can't even hit them back... YMMV.
How so? What prevents PCs from using light to negate such tactics?
I like to follow "rules of common sense". In our game, Fog Cloud results in disadvantage because the target cannot be seen. The weird "advantage for foe in fog cannot see negates disadvantage because attacker into fog cannot see" RAW seems dumb in a Fog Cloud. The target's general location is known unless he Stealths and all attacks are at disadvantage because attackers cannot see. But I could see a DM ruling that unless you know the square, then you have to guess as a decent ruling as well (in which case, one would use the RAW advantage negates disadvantage).