The Old Testament passages were written in a time and place when the common belief was a flat earth, so it makes sense that the writers did believe the earth was flat.
The passage from Matthew, on the other hand, was written when the spherical shape of the earth was well established. However, the Americas were not discovered yet, and it was believed that all the land masses were on one hemisphere, and the other hemisphere was completely ocean. So it is not a stretch to believe that someone on a tall enough mountain could see all the earth's kingdoms.
But even if we accept that, through some sort of divine revelation, that these ancient writers knew the earth was a sphere and that this "flat earth phrases" were just figures of speech, that undermines the claim that the New Testament writers really believed in a literal flood, since, they, too, could have been simply referencing Noah as a popular image rather than a literal historical fact.
But government...is not simply the way we express ourselves collectively but also often the only way we preserve our freedom from private power and its incursions. --
Bill Moyers (quoting John Schwarz)