At the bottom of the page for the Homekit Integration App, under advanced, flip the toggle for Show accessory classes and characteristics, then find your dimmers and in small text underneath each dimmer you’ll see Export as Button, Outlet, Switch, Dimmer and click Dimmer. You might need to refresh the page for the subtext to show up, I can’t remember.
Hubitat is choosing what to export as based on particular characteristics that devices have. Not sure why it defaults the way it does, but it’s trivial to fix.