They're probably referring to the dry powder form, not a solution.  I
have trouble crediting the possibility of a 3% solution exploding, and
if it did, I don't think it would be included in a developer formula.

If it worries you, try this:

Take up the smallest amount of the hydroxylamine sulfate solution
practical in a dropper, and drop a single drop into a small pile (a gram
or so) of potassium carbonate powder.  You'll likely see a reaction,
since the material will probably still decompose, but if it were to
explosively decompose, the quantity is such that it shouldn't be
hazardous.  Do wear goggles, though, just in case; it doesn't take much
of a reaction to scatter the carbonate granules around.