I am no scientist or expert on much, but I have seen cheap Walmart machetes left out in a stump in the PNW woods for a couple months with less rust than that knife has on it.
Looks to me like the knife had exposure to saltwater. I could be wrong.
The knife needs to be seriously cleaned and then protected with some good oil of some sort. There are all sorts of products that will help with corrosion prevention.
Take the sheath apart again and thoroughly clean in inside.
I would also contact Winkler and ask for his advice on this, as he has an excellent reputation when it comes to customer feedback.
Kydex and preventative maintenance is probably the answer to this whole mess.