Even if people did think the Earth was disk shaped, that'd still be wrong.
The Earth isn't spinning fast enough to make that shape. Some things, like galaxies or an accretion disk can be disk shaped, but only because they're spinning fast, and their internal gravity isn't enough to pull it together.

When pulled firmly together with its own gravity, it will form a sphere, not a disk.
If it was spinning faster, the centrifugal force will widen it more and more, until it does become a disk.