Though she says her speech wasn't slurred, you can tell from the story she had speech and comprehension difficulties. Regarding face drooping and arm drift, it could be that she just doesn't remember/didn't notice, again she says herself her memory was failing. I'm sure a doctor would have recognised the symptoms immediately.
She was sitting with her husband and friends for hours and they obviously felt extreme guilt about not realizing she was having a stroke. I'm sure they've all played that party back and forth in their minds for days wondering if there was something they should have noticed. That day would have been analyzed in agonizing detail by all of them. And it sounds like the writer had this conversation, as she had to re-piece so much together.
