I got the impression that he was a poor man's wannabe Evil Kenievel who wanted publicity, and to get it, he had co-opted this flat earth philosophy. It wasn't his original reason for building the rocket. He got into it because it brought him publicity.
That seems to be exactly the case.
'Mad Mike' Hughes, daredevil who built a homemade steam rocket, dies in launch attemptIn the past, Hughes said he believed the Earth was flat, with some publications stating his launches were aimed at seeing the Earth's curvature for himself. In August 2019, however, Hughes told Space.com that his flat-Earth belief was not his launch motivator. He was simply a daredevil, pushing the envelope of homemade rockets, Hughes said.