For procuring iron oxide it's very easy.
When I need it, I make it. It can be bought but me personally I have a hard time paying for rust powder. With a little planning ahead it's easy to do.
Now, in terms of flammability I love this one. People have this preconceived notion that something listed as "flame retardant" is for whatever reason impermeable to flame or spark. This means I can set my tarp or tent even closer to the fire pit! WRONG, WRONG AND WRONG!
Merely this offers more resilience to stray embers, in no way to open flame. Over time this chemical application does indeed fade.
Now we are obviously using volatile chemicals when we apply this homemade solution. If not properly dry, you bet your breeches chances of catching fire are like fireworks on the 4th. However, when properly dry indeed the chances are diminished.
I've said it over a hundred times to people who are interested in any shelter, I absolutely can't stress enough the common sense aspect of having any shelter in close proximity to an open flame.
Accidents and unforseen issues happen. I'm positive they did back in the day as well with shelters and fires. Each individual obviously has to understand the risks involved with anything and exercise good judgement.
Nothing IMO replaces a good head on a man's shoulders. Unfortunately today that seems to be lacking in allot of societies round the big ball of rock and water.