From my very limited knowledge of caramelization, I don't believe his claim that nothing else is added.
This point is very important to why there is so much confusion, and how relatively unknown the effect of DAP on homemade candi sugar seems to be.
DAP is used in at least one method of sucrose purification. Specifically, I have seen at least one article in my internet wanderings that specifically implies the belgian sugarbeet industry uses this method.
Purely as conjecture, it is my belief that if you start with sugar/syrup from the manufacturer that is a byproduct of the crystal purification process...you can honestly state that nothing else was added in the manufacture of the product. Your DAP was already there, leftover from the purification.
As an additional datapoint, I have made this >4 times. I made a 7oz batch with 2TBSP of DAP, and did 4 ramps. I primed a dubble with just this. It is both noticeable and excellent.
240F is too high to return to and get a workably thin syrup at my ambient conditions. Room temps, the stuff is about as thick as lme.
As to process issues I ran into and overcame:
Adding water too fast for cooling can cause the sugar to harden and not dissolve into the water. A royal pain, a lot of stirring, and some potential for burning, all rolled into one.
If you ramp too quick, or don't get a good dissolving initially, you may have a slushy crystally mess up to quite hi temps. Re-add enough water to get down to the 220F range, stir well. It will take a fair amount of time to get back to 260 from there.
The suggestion to go descending max temps in sequential ramps is good. After a ramp to 290, I lost some aroma by going up to 300. I went from toasted marshmallow aroma to more of a rummy caramelly amaretto flavor. Still good though.
Keep it covered while cooling. Loss of water will encourage thickening.
I tried ammonia bicarbonate, but the batch turned out completely bitter. It did smell good, but flavor was disgusting. I may give it another shot, as apparently there are others that have experienced this with only DAP. Any ideas what the confounding factor is for extreme bitter flavor?