It’s probably not 100% necessary; even Ratchet & Clank: Rift Apart (which was designed to use the PS5’s fast loading speeds to switch between worlds on the fly, and supports the latest DirectStorage implementation on PC) can be played off a hard drive, tests have shown. But any PC recent enough to play Starfield on really should have an SSD.
https://www.bbc.com/news/technology-65789916
Apparently it was a story told by an Air Force colonel at an aeronautics conference, who later retracted it.