As far as I know, you are correct. You are not allowed to discharge in state parks unless authorized as part of a DWR hunt, or in self defense pursuant to our states defense laws, but this doesn't prevent you from being armed. It's possible something changed in the latest session of our legislature, but I highly doubt it. I do put a fair effort in staying informed of such laws...
You might want to contact them and point out that their literature violates the state pre-emption and firearms law. Another option would be to photocopy/scan the brochure and send it, along with a letter explaining your complaint, to Mark Shurtleff, our state's attorney general.
This has been a semi-common issue, with many local parks and other government land being posted as not allowing firearms. All such signs are erroneous, and either the agency was not aware of the law, or has just not gotten around to changing their literature.
If you do a little searching at utahconcealedcarry.com you will find that there are a number there who have had to do the same thing to get such signs removed. The latest I am aware of involved signs at Wheeler Farm.