Can a manufacturer also be an open-source software steward?

Yes, a manufacturer can also be an open-source software steward.

This can happen whenever a manufacturer releases open source software and meets the requirements to be the open-source software steward of that project.

A manufacturer of commercial open source software can even be the open-source software steward of the community edition of the same project that it commercializes. In such a case, it has manufacturer obligations to its customers and steward obligations to the users of its community edition.