The Bible says that the earth is round. That doesn't mean they are saying it's flat. That's what people are basing the Bible saying that the earth is flat by.

If they're saying it is flat than how did they know it was round?

because of the moon and other things!

Wouldn't you agree that you can see the sphere shape in the moon in some nights?

Especially in some parts of the world...

So if they base the earth being round by seeing something like they moon, then they must of also assumed it was a sphere because they can see that concave in the moon.