Vaguely, caps are often not rated to withstand more than 200 degree environments, electrolytics and film. Of course there are exceptions, and higher temp caps. Also, this is usually rated lifetime. LIke, 10,000 hours at 105C or something. The cooler they stay, the more life. The hotter, the faster they can die. Using high temp caps also means longer lasting in the same circuit.
I think this is very much in the scale of temperature we are talking about. Even low temp solder can’t melt until it reaches 360 degrees or so. So yeah, holding a soldering iron to the leads for a while can really damage a cap. I usually have my iron around 620-650 depending on how much metal has to heat up, BUT....
The time of exposure matters a lot. A quick hot iron can expos parts to less total thermal energy than a slow one, and therefore the internals of the cap will heat up less.
Anyway, this is all theory. :) The fact that the cap that failed was in fact exposed as I was worried about kind of explains it in my mind. :)
The cap is dead anyway. Try again and make sure you can solder fast. Lesson learned.
Best,
Erik