Round steering wheels would still make sense on a flat Earth, and as others have mentioned a disk is both flat and round; Flat Earthers tend to believe either that the Earth is disk-shaped and with an edge that you can fall off, or that it is infinite in all directions.
With that being said, with regards to comparing the Moon and the Earth:
A pretty central part of many Flat Earthers' beliefs is that the Earth is unique in the cosmos. Based on what I've seen, a belief in a Flat Earth all but requires that you believe in a creator deity, who set the Earth apart from all other things in the Cosmos. Many (but certainly not all) Flat Earthers accept that the Moon and/or Sun and/or planets are spherical, but to them that is not relevant, because the bodies which inhabit the celestial dome are of a completely different nature than the world on which we live.
I am certainly not advocating this view, but it's something that needs to be remembered when discussing Flat Earthism.