The terms "flat" used in the context of the universe and "flat" used in the context of the Earth refer to different concepts, and the usage is not a result of stubbornness or an attempt to discredit flat-Earthers.
When astrophysicists describe the universe as "flat," they are referring to the overall geometry of space on a cosmological scale. In this context, "flat" means that the spatial geometry of the universe follows the rules of Euclidean geometry, where the angles of a triangle sum up to 180 degrees, and parallel lines remain parallel and never intersect. This flatness relates to the large-scale structure of the universe, not to the shape of individual celestial bodies within it.
On the other hand, when people discuss the shape of the Earth, they often use the term "flat" in the colloquial sense to mean a two-dimensional plane or a surface without curvature. However, the scientific consensus, supported by extensive evidence, is that the Earth is not flat but rather an oblate spheroid—a three-dimensional object with a slight bulge at the equator and a curvature consistent with its spherical shape.
So, the term "flat" is used differently in these two contexts due to the scales and concepts being discussed. The concept of "flat" in the context of the universe refers to the overall geometry of space on a large scale, while in the context of the Earth, it refers to a two-dimensional surface. The scientific understanding of the Earth's shape is based on a wealth of evidence from various fields, including geodesy, satellite measurements, and global navigation systems, which consistently support the Earth's spheroidal shape.