I don’t think flat-earthers are capable of indulging in concepts like other planets. They’re entire personalities are completely dedicated to indulging in their silly “theories” and insisting that it’s important.
For example, have you ever asked a flat earther what possible incentive could there be for “the government” to lie about the shape of the Earth? The only answer they’ve ever given me is “control”, then they get very smug as if this someone proved something and won’t really discuss it any further. I don’t think they’re capable of imagining anything beyond that.