This seems to conflict with the info you posted, and at the moment, I have no way to resolve which, if either, is correct. I suppose that one of us could run a sample up to temperature and see what happens.
What part of "I use it and it works" escaped you? I don't know why your link lists the temp it does, but something's not right there. Some research on salt bath annealing or salt bath heat treatment will show you otherwise as well.
It doesn't boil at 752F, not even close. I use it at 900F for both annealing brass and heat treating stainless steel (that is 900 per a K type TC and PID temp controller, not some random thermometer). The chart I posted and others I found all said 930 F as the maximum working temp for potassium nitrate; I haven't found any reason to doubt that, and the boiling temp may be even higher.
As to the other one - sodium nitrate's listed max working temp is only 2 degrees lower than the 60/40 mix you're saying we have to use. Both are way higher than needed for annealing.
Edit - both of these salts do have a tendency to fizz a little bit and create bubbles on the surface as they melt from ground particles - that can happen around 750 F and maybe some researchers are mistaking that for the boiling point. Once it settles down though, both of these can be used up to at least 900 F in my experience, as I said above.
