FPC doesn't sound right since you can have a react component for just a single part of a page, it doesn't necessarily need to take control of the whole page.
Yes of course the components are usually not the entire page; don't take the terms too literally. I'm still searching for the best name for these modes ("sprinkled" isn't quite right either). Although "full body component" or "big section component" might be a little better, I suggestion not missing the underlying idea or point due to imperfect terms.
Very true, and is a highly useful tool to add interactive content to a section of an MPA - React used in this way, without requiring a state library and everything else is actually quite nice to work with.