Systems Manager does indeed support Shared iPad for Apple Business Manager devices.
One of the main difference currently between Apple School Manager and Apple Business Manager is there's no way to sync the Owner accounts in SM > Owners page for ABM (where there is an API to do this for ASM managed Apple IDs). Also as you'd expect, ASM can only use the Classroom app and Education profiles to configure the classes. However, Shared iPad does indeed work for Apple Business Manager in Systems Manager, just without those school features.
If you enable "Shared iPad" in the DEP settings from Systems Manager for an Apple Business Manager device it will setup as a Shared iPad device upon initial setup. From there you can sign in with a managed Apple ID from the business.apple.com account or use a guest temporary session to sign in.