According with these Minecraft Wiki blast resistance and explosion causes charts, the smooth quartz block and the smooth quartz stairs should have a blast resistance of 6, which is more than enough to resist the ghast fireball attack. But I've been attacked by one and it destroyed a wall made of that block and also the stairs of that type. The only exception that can effectively resist this attack is the smooth quartz slab, which blast resistance is also 6.

I don't believe now that this could be an error of any of the Wikis information, it's a bug at any light.

Anyone can reproduce this by building a big wall made of smooth quartz block, along with some smooth quartz stairs and slabs, and spawning a Ghast; this can be done whether in the overworld (using a spawn egg in the creative inventory) or the nether but since the environment is the nether it is recommended to be there. Then change the game mode to survival and be spotted by a Ghast and make it to hit the building with one of its fireballs.

PS: New Minecraft Wiki links updated.