I had the same questions. Then I read the last sentence of the article.
"...uses a small rocket engine for the final orbital insertion."
Ah. Yes, lifting a rocket the first few miles makes for the bulk of rocket mass. Hence Virgin Galactic’s approach of a large airplane doing the initial ~5 mile lift; much less mass needed when launching from up there.
Do though wonder then what the rocket’s mass is for SpinLaunch. That still leaves very little mass for satellite itself.