That does seem realistic considering how often the topic pf flat-earthers comes up in discussions on social media, despite it being an extremely niche view.
OP suggested that 10% of people think there are more than 50% of flat-earthers when the real number is 0.1%. Phrased another way, 10% of people expect half the people they meet will be flat-earthers when it's 1 in 1000.
That does seem realistic considering how often the topic pf flat-earthers comes up in discussions on social media, despite it being an extremely niche view.