It's cOmPLiCaTeD
Anyone who has ever written a proposal probably knows what I’m talking about here. A grant proposal has to be based in reality and evidence - you need to demonstrate (through evidence and, e.g. preliminary data) that you can perform the work. And you need to write an introduction/background that is about what is known in the field and specifically for the problem you’re addressing. That’s all (arguably) non-fiction.
Then comes the fun stuff. After you establish the gap in knowledge that you’re going to address you have to jump that gap in to the realm of speculative fiction. How will you address the gap? Well, you’re essentially writing hardcore science fiction. Is it based in evidence and things you could do? Sure. Is it also a fantasy about what you hope to accomplish, how things will work, and based on whimsy and flights of fancy? Also, probably yes.
At the end you hope your readers (that is, the grant reviewers) will be engaged in your story and will be excited by the fantasy world that you spin - so much that they want to go there themselves, or at least, provide the funding to send you to that magical land. Will the land be as fantastical when you’re actually journeying through it? Probably not. Will the journey be as linear as you make it out be in your work of fiction? Nope - not a bit. Will you discover new lands and make new (conceptual) friends along the way? You bet!
Adventure on my friends.