This roughly tracks with my own thinking on the scenario. I don't understand the perceived danger.
This is a Dad who also frequently goes to playgrounds. Tbh, in my experience, most moms are super kind and generous to a man who's out alone playing with his kid because it's the sort of thing they want to encourage/reward.
The only times I've ever felt discriminated against as a male parent by female parents is in group play settings where the women form a clique and don't really want you to talk to them, but even then they're usually mature enough not to have the kids feel any of this, and nobody owes me letting me socialize with them, so it's whatever.
The danger was perceived precisely because it's rare and uncommon and the whole thing unusual. It's the only time I've every personally encountered something like this, so it made me believe that this woman knew exactly what she was doing and we interpreted her words as an insinuated threat. Why else would she say something like that about a man who everyone could clearly see was just trying to help? No, there was no confusion. Whatever she was up to was malicious.
Because it's so uncommon is why my dad was even going around trying to help this girl in the first place, because he never imagined something like that happening. But then we got a hint of it, and decided to just disengage and not risk it.
I understand disengaging in the situation, sure.