About the name: yours is the official Posit one, and you were there first, so I'll rename my branding; there should be one ggsql, and it's yours. Mine only exposes VISUALIZE as the keyword anyway.
The actual name of the extension is the-stats-duck, which runs inside duckdb-wasm (it powers an in-browser data tool) and emits a Vega-Lite spec inline for the host to render. Your implementation (which I think is Rust based and an in-process HTTP server that opens a browser), is a native pattern, but correct me if I'm wrong! mine is deliberately thin and wasm-safe, not a whole engine.
About the parity, you're right, and I'm not chasing it; for real grammar-of-graphics, ggsql should be the tool! but, if that's ok with you, I'd love to keep the syntax aligned!