One thing to consider with the flame retardant is that while it may be considered non-toxic on contact in normal conditions (normal use for this product), they probably did not forsee their product being constantly heated on a snake rack. The constant heat may (or may not) release toxic vapors from the retardant.
Many chemicals have low vapor points and may be relatively nontoxic as solids but are much more dangerous as vapors. While it may be safe, I would definitely not use this flame retardant on a heated rack. A PVC rack is the way to go if you are concerned about fire. The cost difference really isn't that much if you are using quality materials in a DIY project.
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