Why is this is a map error?
How is this NOT a map error? 
Its not unheard for minerals or gems which are generally found deep within the earth to have exposure on the surface.
This isn't the first report of Cotton Candy having a spiral reaching the surface, but its not really that common.
It is most definitely an error. If you try and re-gen that world and embark on that same exact area, you will not get the spires. The exposed spires and even occasionally exposed magma sea, seem to be related to something in memory not getting cleared properly causing oddities in worldgen. If this was something which could be consistently repeated I would guess that it could simply be worldgen creatively applying minerals, but since it is completely random wether your world will have these spires, and completely unreproduceable on the same worlds, it is most definitely NOT normal behavior for the game.
On that note, I would embark there and not consider it cheating. If you don't turn off invasions things should get pretty interesting right quick once you start mining the bluemetal. Also, my personal experience with this was a map with 2 spires that extended 15 z levels above the floor and were hollow all the way to the bottom. I decided to see if there were any demons at the bottom so the first thing I did was went down the tubes with some staircases. It turned out that hell was completely deserted for whatever reason.