How is this different from Google’s Rich Results Test (search.google.com/test/rich-results)?

This tool parses JSON and applies an embedded registry of 15 top-level profiles. It does not query the current Schema.org vocabulary, recursively validate all graph nodes, check every enum or resolve entity references. Google's Rich Results Test addresses currently supported Google features; Schema.org Markup Validator addresses broader vocabulary markup. Use the tools for their different purposes. Google no longer shows FAQ rich results, even though FAQPage remains a Schema.org type.

References: Schema.org Markup Validator; Google Search documentation updates.