Hi Daniel,
"I do realize that hard solder makes a much stronger and higher temperature joint, but at that nipple's location, away from any flame and only likely to experience steam temperature, I would think that a soft solder joint would work just fine. Am I missing something, or are you simply being certain that everything is done as right as rain?"
A soft soldered nipple would be more than adequate and possibly originally used by Stuart Turner. It would have been left had the solder not boiled during the annealing process. The nipple was hard soldered as it is my default when soldering all boiler fittings, possibly because in the days of gainful employment I rarely used anything else.
Have a great Christmas and a Healthy New Year.
Take care Tony.