Can a solo maintainer be considered to be an open-source software steward?

No. As defined in Article 3(14), an open-source software steward must be a legal person (e.g. a company, an organization, etc.) in contrast with a natural person (i.e. a human being). The obligations of open-source software stewards described in Article 24 therefore do not apply to solo maintainers. It is worth noting however, that natural persons are subject to the same obligations as legal persons would be should they monetize their poject.