It's an intermediate step. Starfinder is to PF2, what Book of 9 Swords is to 4E.
I think a better comparison is Star Wars Saga. SW Saga was fundamentally a 3e-based game (though more akin to d20 Modern than D&D), but it served as a test bed for some of the concepts that later showed up in 4e (mainly every offensive thing being an attack roll instead of a save, and broad level-based skill competence with training giving a bonus on top). Similarly, Starfinder is primarliy PF1-based, but a few concepts that also show up in PF2 show up there.