The description of the heavens being stretch out like a curtain to form a tent over the earth should show you that the earth is being described as a circular disk, not a sphere. Martin Luther did not teach a flat earth, however
Cosmas Indicopleustes certainly did in the 6th century. He advocated that the church stop compromising with the pagan Greek science of spherical earth and learn what the bible teaches about the shape of the earth without being unequally yoked with unbelievers. He was
Sola Scriptura long before Luther and would have considered Martin Luther inconsistent and serving two masters. If you want to go for real
Sola Scriptura, YEC and geocentrism simply don't cut it. The church's compromise with extrabiblical science began long before Copernicus.
And quite right too.