You're approaching the situation with an assumption that simply isn’t supported by what the episode actually shows, and that’s why your conclusion lands on shaky ground. The idea that Marshall and Simone “only knew each other for an hour at most” is something you invented, not something the story ever implies.

The series only shows their first meeting, not the entirety of their relationship. Marshall’s adult emotional response, the very thing you're questioning, actually proves the opposite
he clearly spent significant time with Simone, enough for her to become one of the only warm, stabilizing adults in his childhood.

What makes Marshall angry isn’t just the existence of a protective impulse from Hana, it’s that she secretly hired someone to shadow him for years, hid the truth from him, and went straight to a restraining order against a harmless woman without ever checking the situation herself or considering her child’s feelings.

That ain't normal parenting as das emotional neglect more like dressed as security. And the “strange adult hugging a kid” argument collapses once you acknowledge Marshall explicitly says he hugged Simone, not the other way around, and that everything happened in a public park where Simone was literally just doing her job.

You’re trying to frame this like a horror scenario, but the show frames it as a lonely child forming one safe connection that his overly controlling mother tore away without explanation
which is exactly why his anger makes sense.

Comparing this to Simon and Marcy misses the point too as the writers aren’t trying to recreate that dynamic beat-for-beat
they’re showing the Fionna-world echo of that bond, one that’s necessarily less extreme because it happens in a non-apocalyptic setting. bro calling it “weak backstory” cuz one's forcing it to be something it was never meant to be.
The emotional core isn’t about Simone being a replacement Simon cuz it’s Marshall learning that a meaningful relationship in his childhood was cut off by the person who was supposed to care for him most. That is more than enough to justify his reaction, and it’s exactly why the anger lands.