It's been known and taught for many years that the earth is not a perfect but an oblate spheroid
Correct. The slight polar flattening is caused by the Earths rotation. This is evidenced by the fact that the gas giant planets, which rotate faster due to higher mass show the flattening effect more than the Earth does. Therefore the faster a planet rotates the flatter it will become. With a rotation period of 24 hours, the Earth spins far too slowly to become flat.
Geological records in rocks show that the Earth rotated much more rapidly in its early history. That means the flattening effect for Earth would have been more pronounced in the past. So as it slows down, it is getting less oblate spheroid and more spherical.