1. You don't have exterior parts for your house which fit the sci-fi theme. If you don't have exterior parts which match the theme, you won't get many points no matter how good your interior is.
2. The guide you are looking at may be inaccurate. I use Thonky a lot, but not everything there is true, so you might want to try to find another source just to make sure sleek furniture is sci-fi.
Also, remove any furniture which does not match the theme from your house as you get the most points if all of your furniture matches the theme.
I think every room is rated independently of each other, so I don't think filling every room with a complete sleek set would be a problem. I had the same set in two of my rooms to get all the golden exterior parts. It's possible that only four of my rooms were enough to get everything, but I think every room contributed to my point total since I didn't have that much furniture in each room.