I think Geneva is spot-on, though, when suggesting that the story is also wider and more universal than that.

I agree, and I take it as a tribute to Tiptree's skill, but I do see gender as pre-eminent. That it's an argument about power imbalance doesn't mean it's not specifically about power imbalance as distributed across genders. Similarly, though I could describe my reaction to the ending as 'being disconcerted because the power I take for granted is suddenly irrelevant', and it would be absolutely true, it is more accurate to say it throws me because the power I unconsciously associate with my gender is suddenly irrelevant. It demonstrates just how much it's not irrelevant in day-to-day life.