Flux does not need time to work. It works instantly with heat. That being said, it needs to KEEP doing its thing during the soldering process, and if you heat too long, it'll burn off and stop working.
Oxides are formed when metal (copper) reacts with oxygen. These prevent solder flow and prevent bonding btn copper and solder. Mechanical removal via sanding is good for getting most of the oxidation off of the copper prior to heating.
The problem is, heating, itself, causes oxides to quickly form. That's the reason we flux. It prevents oxidation.
The reason you shouldn't overheat a joint is bkz it'll burn all the flux away which will allow the oxides to form unfettered.
So, my guess is that yes, using a super-hot torch means your risk of burning out the flux is higher. That being said, flux doesn't burn off THAT FAST, so you're probably ok, unless you're getting the joint white hot for a long time. If you heat it as quickly as possible, remove, then solder, you're likely fine.
In addition, flux burns at a lower temp than solder, with the El Cheapo tip, it's easier to improperly heat, causing the joint to be just hot enough to run off all the flux, but cold enough to not let the solder melt.