It's actually worse than that. The Sigma MC-21 EF to L-mount adapter only supports single-shot autofocus. It does not support continuous autofocus (AF-C) in video mode at all. So you can't even try it, even if you wanted to:Since the camera is already shipping, new (non-sponsored) footage is trickling out. Some reviews of the video AF are pretty brutal, even with the kit lens. With an adapter, fuhgettaboutit.
https://www.sigma-global.com/en/lenses/cas/product/accessories/mount-converter/#mc-21