This. We have only two real options regarding the nature of her immunity:
1: She is genetically immune, which means the lethal vivisection they were going to commit on her was brainlet nonsense; all they'd find in her skull is a normal fungal strain that was incapable of functioning properly due to her weird body chemistry. Making her into a breeding sow and playing a generations-long eugenics game is their only actual option in this scenario.
2: She has a broken, mutant strain of the fungus that fails to launch past the earliest part of the lifecycle, while also preventing regular strain infections from being able to set up shop since the tissues are already occupied. They also outright state that just by looking at fluid samples from her, she has just as much of this infectious shit in her bloodstream as any normal infectee, so they don't need to drill a hole in her head to get samples. In this case, spreading her busted form of the infection AS a vaccine in and of itself would be elementary, as the first round of fellow infectees can then act as blood sample donors or whatever and so on, thus spreading it exponentially.
Either way, they were going to either kill the only known person with genetic immunity to the infection instead of making her a breeding sow, or pointlessly kill her for a fungus sample that they could have gotten just as easily by having her spit into a petri dish.
It was dumb and everyone involved with writing the ending needs slapped.