Absolutely correct.

My house is 100 years old, and I just learned during a foundation repair that they commonly didn't even pour footings. They also didn't properly compact the ground, so sagging soon after construction is pretty common from what I have heard. Concrete doesn't last forever....

You can cut shims from 2x4s and then lay plywood on top of them to create a level floor. This will raise the height of your floor slightly. You could also use leveling compound, but for that amount of slope you will have a lot of buildup on the downhill side, and it says heavy as concrete.

You could use leveling compound with filler pieces of cement board to make the process go more quickly. Cement board might have air pockets in it, so it'll be a little lighter.