That is not at all clear. Look at the data there:
Compare that to the figure you’d expect for young earth creationism. There certainly is a religious version of flat-earthism, but there is also a non-religious version. That is not true of YEC.
The article has some more interesting information:
Just 66% of millennials firmly believe that the earth is round
Those who believe the Earth is flat vary in the exact theories, but whether they believe in science or religious literature as the basis for their claims, a new YouGov study reveals that 2% of Americans resolutely say the earth is flat.
Flat earthers find traction in their beliefs among a younger generation of Americans. Young millennials, ages 18 to 24, are likelier than any other age group to say they believe the Earth is flat (4%).
Notice that this age group is more likely to be a None. There is good reason to think flat-earthism is associated with conspiratorial thinking and a general distrust of institutions.
These results, regardless, need to be taken with a grain of salt, as they are based a self selected group.