Just another silly question that may deserve a wise answer.
According to Wikipedia a flat universe would have this property:
Zero curvature (flat); a drawn triangle's angles add up to 180° and the Pythagorean theorem holds...
This is not the case for the round Earth which has positive curvature on its surface:
Positive curvature; a drawn triangle's angles add up to more than 180°...
The spherical objects in the universe may make it appear bumpy from a local perspective. From a more global perspective one would view the universe as flat having zero curvature even though those spherical objects within the universe have positive curvature.
"Shape of the universe" Wikipedia https://en.wikipedia.org/wiki/Shape_of_the_universe