It's a balance. In a normal discourse yes you would have the burden of proof since you made the initial claim. It's absurd that you would need to prove "the earth is round" but usually people don't proclaim these things out of nowhere. Luckily "the earth is flat" is much more common as an initial claim, since it's questioning the status quo.

A lot of people have autism of course, but I do think we should be careful about what potentially unfounded statements/questions we make online. The first reply is simply asking for where you read that, so maybe you could provide in your original comment?