Things of note:

-She didn't like the story
-•She found a mismatch between the 'silliness' of the sandbox versus the tone of the story.
-She loved it otherwise

•This should say something when a "non gamer" is making very astute observations about the fundamental nature of immersion in games, while the mainstream press never ever thinks in these terms

That is kind of the shtick of her entire channel, and it’s fantastic. I recommend checking her other things out, especially the two videos she’s done on Red Dead 2, and the thrashing she gave Jedi: Fallen Order, in case you start to think she’s completely naive about the game industry.
Also her game awards video from this last year was great.

Has she played other Halo games before, or was this her first?

The point about the mismatch between gameplay and story is not something I would have ever considered. That might be different if Reach was the first Halo I’d played, but I’m so bought in on Halo already, that it didn’t really stick out when I first played it.