@thomas-young You have it kind of right and kind of wrong.
You are right when you say:
"The key factor here is the camera's line of sight.
When the line of sight is perpendicular to the picture plane, it will be a one-point perspective, regardless of how high or low the horizon line is."
You are also right when you say:
"For a two-point perspective, the line of sight needs to be rotated upwards (or downwards), not just lowered."
Yep. Totally agree.
But you are incorrect to apply that concept to this particular illustration.
The OP @ChloeGreenbergArt specifically wrote:
"Drawing from a bottom-up perspective like this is very out-of-my-element, but it’s important for the story."
That suggest that Chloe is looking for a "worms eye view" or a "low angle POV". In that scenario the camera is placed low to the ground and is looking up at the subject. This means that the camera is "rotated upwards" resulting in a two point perspective system.
Thanks for the opportunity to further clarify when and why to apply different forms of linear perspective. That's exactly what these forums are for!