how to level uneven plywood floor for tiling?
You did hear correctly: the floor must be as flat as possible (and as rigid) in order to support the tile. You mentioned the floor being supported on posts. If there is enough head room  a better repair is to shim-up the low spot from below. Depending on how severe the floor depression is this may entail hammering some store bought shims under the joists or installing concrete footings so as to support new posts.

The quick easy solution is to outline the low-spot  of floor , mix-up a bag of self leveling floor mix, and trowel it into the depression feathering the edges.

It would also be a smart idea to check the other sections of floor that will get covered by tile for any squeaks, bounce or shifting. Nail or screw to secure them to joists. 

When you are ready to begin installing the cement board it would be wise to embed the 3' x 5' x 1/2" cement boards in a thin slurry of thinset for complete support. Use cement board screws to affix them to the floor.