They shouldn't. Much less should they believe the headlines. As the article says: "nine percent of young adults answered that they have “always believed” the planet was flat."
Of those 9% what fraction answered this way as a joke. Given that 1 in 10 are sociopaths anyway, a large fraction of those must be a joke.
Then there are those who figure any question like this is a trick question and that the "obvious answer" is wrong. Actually it is wrong. The earth is not round as in a sphere, but slightly flattened on the poles and bulging out at the equator.
Then there are the incredibly ignorant and if it is 4 or 5% well, that's life and you don't fix that in America or England. Socialists might get a "better" result only because socialists never question what their socialist government tells them.
As the article says: “nine percent of young adults answered that they have “always believed” the planet was flat.”
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Example Mueller perjury trap question. Include “always” in the question. If you answer “yes”, the trap is sprung. Most very young children would have an easier time thinking the Earth flat instead of being a sphere, a big ball you would fall off.