I could build that!
Well, why not ?
No, that floor isn't sloping. There's actually storage under that angled part there in the back. Probably not real useful but it is a small building and there wouldn't be much space wasted.
No, the floor is level. I can find some picture of the interior but it is a little confusing. I think you can probably imagine. . . yeah, the ceiling slopes. And, it is fairly spacious in there.
Kind of eliminates the need to be concerned about roof slopes, tho'. I mean, the thing is a b.o.x! It would need to be kind of a high box because the roof would be sloping down to the back and there is a need for head-room. But - Imagine the view you'd have from your little garden office by elevating the floor like that!